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Extension > Yard and Garden News > June 2014

Friday, June 27, 2014

Squash Vine Borers are Out Now

Jeffrey Hahn, Extension Entomologist


Jeff Hahn, Univ. of MN Extension

Photo 1: Mating squash vine borer adults

Squash vine borer adults have just been sighted. If you are growing summer squash, winter squash, or pumpkins, take the proper precautions to protect your vine crops from this insect. This garden pest less frequently attacks butternut squash, cucumbers and melons. If you are unsure whether squash vine borers are in your area, monitor your garden for their presence.

Set out a yellow pan of water. Yellow is attractive to squash vine bores, and they will fly to and fall into the water. Check these traps at least once a day. Squash vine borer adults are easy to recognize as they are wasp-like and about ½ inch long. They have an orange abdomen with black spots and the first pair of wings is metallic green while the back pair of wings is clear. These moths are active during the daytime and you may also observe them when you are in your garden.

Management of squash vine borers is often challenging. There are several steps you can take to try to minimize them, such as planting less attractive crops, making a later, second planting, using row covers (note: do not use row covers if planting vine crops in the same site in consecutive years), and well-timed insecticide treatments. For more information on squash vine borers, including management, see Squash vine borer management in home gardens

Don't be Fooled by Fungus Killed Flies



Dori Eder

Photo 1: Fungus killed flies on grape. Despite any circumstantial evidence, they are incapable of damaging plants.

People are finding a curious insect in their gardens and yards now. Grayish black flies, about ¼ inch long, are being found clinging to the leaves and stems of a variety of different plants. Their legs and wings are typically splayed out in odd, unnatural positions. If you watch the fly carefully, you will notice it doesn't move. That's because it's dead - it has been killed by a fungus that is specific to these flies.

This fungus has been particularly common this year due to the abundant rainfall we have experienced throughout much of Minnesota. When seed corn maggot flies become infected with this fungal disease, they usually fly to a plant or other object and climb up. Eventually they die, leaving their legs and wings in whatever position they were in at the time of death. Additionally, their mouthparts are often extended out. In recently infected flies, the abdomen is swollen and whitish, with fungus protruding between the body segments.

When these flies die on a plant that is damaged, it seems reasonable to blame them for this injury. However, seedcorn maggot flies do not feed on plants (in fact there are no adult flies in Minnesota that directly feed on plants). They are just a curiosity and should be ignored.

Blossom blight blasts crabapple blossoms

Michelle Grabowski, UMN Extension Educator


M. Grabowski, UMN Extension

Photo 1: Brown blossoms and leaves infected with fire blight clinging to a crabapple tree

This June, many gardeners were surprised to see the blossoms on their crabapple trees turn brown and shrivel along with the leaves growing alongside them. In many cases blossoms and leaves became discolored all along several branches (Photo 1). In other cases individual clusters were affected. A month later, these brown blossoms and leaves remain attached to the tree and are likely to stay through the growing season. These unattractive brown clumps of foliage not only affect the beauty of the tree but have a significant impact on the progression of the disease.

These unusual symptoms are caused by the disease fire blight. Fire blight is caused by the bacteria Erwinia amylovora, and it affects trees and shrubs in the Rosaceae family. Fire blight can often be found on crabapple, apple and mountain-ash in Minnesota. Serviceberry, raspberry, cotoneaster, and hawthorn are susceptible but less commonly infected.

Although blossom blight is a rare occurrence in Minnesota, fire blight is not. The fire blight bacteria can infect young green shoots and leaves in addition to blossoms. In order to start an infection, the fire blight bacteria need warm wet weather (110 degree days above 65F to be exact) and a wound or natural opening. In a typical Minnesota spring, temperatures are too cool during crabapple blossom to allow the fire blight bacteria to multiply and start an infection. When weather warms up later in the season, fire blight often infects young shoots that have been injured by insect feeding, wind whipping or other factors. This year early warm wet conditions allowed the bacteria to blight blossoms in addition to shoots.

Regardless of how the bacteria first infect the tree, once an infection has begun, the fire blight bacteria can move through the infected blossoms or shoots and into the adjoining branches. With time the bacteria can move into the main trunk and even into the roots of the tree. Infection of the trunk or roots is lethal but infection of blossoms, shoots and branches can be pruned away if caught in time.


M. Grabowski, UMN Extension

Photo 2: Purple red discoloration of bark where the fire blight infected blossoms attach indicates progression of the disease into the branch. Healthy bark of this cultivar is yellow green

Gardeners that have observed symptoms of fire blight in their crabapple trees this spring should monitor trees throughout the growing season. The extent of the progression of the disease through the tree will vary depending on the resistance level of the tree. In highly resistant trees, the infection may not progress beyond the blossoms. In less resistant trees the infection will continue into the adjoining branch causing a canker. In highly susceptible trees, the infection can progress rapidly through the branch. Home gardeners can monitor the progression of the infection by looking for purplish brown discoloration of the bark (Photo 2) and continuing death of leaves along the branch.

Infections within branches should be noted for removal during the dormant season. February and March are ideal times to prune out branches infected with fire blight because the bacteria are not active at that time. Pruning cuts should not be made during the growing season (due to risk of the pruning wound becoming infected) unless the infection is close to infecting the main trunk. This may happen if suckers or water sprouts became infected or if the tree is highly susceptible. In that case, the branch should be cut 8-10 inches below the discolored bark on a cool dry day. Pruning tools will need to be sterilized after every cut with rubbing alcohol, a 10% solution of household bleach or Lysol®. Trees infected with fire blight should not be fertilized this summer as this will promote young succulent growth that is highly susceptible to infection. More information about fire blight can be found on the UMN Extension Yard and Garden page.

Thursday, June 26, 2014

Rose Chafers are Here



Cindy Schmid

Photo 1: Rose chafers on peony flowers.

Rose chafers are out in full force right now. If you live in an area with sandy soil, you are much more likely to see them in your garden. These scarab beetles feed on a variety of plants but are particularly interested in the flowers of roses and peonies. They can also feed on the leaves of a variety of plants. Rose chafers feed for about three to four weeks; we can expect to see them into early July. There are several options for dealing with this beetle including handpicking and the judicious use of insecticides. For more information, see Rose Chafers.



Thursday, June 19, 2014

Start Trapping for Spotted Wing Drosophila

If you grow raspberries, strawberries, cherries, blueberries, or other susceptible soft-skinned fruit, start a trapping program to monitor the potential presence of spotted wing Drosophila (SWD). A single SWD has been detected on June 6 in a trap set on the University of Minnesota St. Paul campus. Last year, the first SWD was trapped on June 27. It is safe to assume that SWD are present throughout the state.



Jeff Hahn, Univ. of MN Extension

Photo 1: Use traps to detect spotted wing Drosophila's presence.

Home gardeners can monitor SWD with homemade traps. Use a large plastic cup with a cover and make several 3/16th inch diameter holes near the top. Put one to two inches of apple cider vinegar into the cup. Add either a yellow sticky card slightly above the vinegar or a little bit of liquid soap, such as dish soap. Hang traps on branches in a shaded location near fruit. Check traps at least once a week, replacing the sticky card (if used) and apple cider vinegar bait. Dispose of the old apple cider vinegar away from the trap location.

Early detection is very important when dealing with SWD as they can rapidly reproduce to large numbers and damage fruit. Management is a three pronged approach, monitoring (trapping), sanitation, and insecticide treatments. For more information about SWD including management, see Spotted wing Drosophila in home gardens.

Monday, June 16, 2014

June 15th Issue of Yard and Garden News

In this issue:

Damaged Lawns: Steps to Bringing Some Life Back

Don't Confuse Six-spotted Tiger Beetles with EAB

Cabbageworms are Active Now

Protect your Apples from Plum Curculio - Now!

Two Early Flowering Plants for Bees

Two Early Flowering Plants for Bees

Karl Foord, Extension Educator - Horticulture




Karl Foord

Photo 1: Andrenid bee approaching Golden Alexander's flower



Karl Foord

Photo 2: Andrenid bee on Golden Alexander's flower




Karl Foord

Photo 3: Two different Andrenid species on Golden Alexander's



Karl Foord

Photo 4: Bombus griseocollis on Blue False Indigo





Karl Foord

Photo 5: Bombus griseocollis on Baptisia australis



Karl Foord

Photo 6: Brown-belted Bumble Bee queen on Blue False Indigo

One of the keys to have a great pollinator garden is to have different types of flowers available for the pollinators at different times of the year. Two early flowering favorites of mine are Golden Alexanders (Zizia aurea) and Blue False Indigo (Baptisia australis).

Golden Alexander's flowers are shallow and easily accessed by small bees with short tongues such as the Mining Bees (Andrenid spp.) shown in photos 1 - 3.

Blue False Indigo flowers are much more difficult to access by small bees but are easily accessed and preferred by bumble bees. The bee in photos 4 - 6 is a Brown-belted Bumble Bee queen (Bombus griseocollis).

Protect your Apples from Plum Curculio - Now!

Karl Foord, Extension Educator - Horticulture



Karl Foord

Photo 1: Plum Curculio feeding on young apple



Karl Foord

Photo 2: Plum Curculio feeding on young apple fruit



Karl Foord

Photo 3: Plum Curculio feeding hole



Karl Foord

Photo 4: Apple Maggot Barrier protecting young apple fruit from Plum Curculio

The Plum Curculio (Conotrachelus nenuphar) is a pest that may be active now in your apple trees (Photo 1). The first damage caused by these insects is feeding (Photo 2) which creates a small hole (Photo 3). Although this hole is shallow (usually less than 1/8th of an inch, multiple such wounds can affect the quality of the fruit. One way to avoid this type of injury is to place nylon stocking maggot barriers on the fruit (Photo 4). See "A Different Way to Protect your Apples from Apple Maggot". Soon these beetles will mate and you will begin to see a different type of damage caused by the female cutting a crescent -shaped slit into which she will deposit her eggs. See photo 3 in this Plum Curculio Insect Identification Sheet.

The actual size of a plum curculio is @ 1/4 inch long, and the size of the apples in the photos is @ the diameter of a quarter.

Friday, June 13, 2014

Cabbageworms are Active Now



Jeff Hahn, Univ. of MN Ext

Photo 1: Watch for cabbageworms on your cole crops.

Are you growing any cole crops, such as cabbage, broccoli, kale, radish, or turnip, in your garden? If you are, it's a good idea to check them out for imported cabbageworms. As an adult, imported cabbageworms are pretty white butterflies. As a caterpillar these larvae are light green with thin yellow stripes running down their body; a series of short hairs gives it a velvety appearance. It's the caterpillar stage that damages plants by chewing holes in the leaves, sometimes seriously defoliating them.

These crops can tolerate some feeding. However young seedlings and transplants are particularly susceptible to feeding injury and should be protected. Checking plants regularly is very important so any infestations can be spotted quickly to minimize injury. If any imported cabbageworms (and cabbage loopers later in the summer) are discovered, there are several options for dealing with them, including handpicking and using the low impact insecticide Bacillus thuringiensis. For more information, see Caterpillar pests of cole crops in home gardens.

Don't Confuse Six-spotted Tiger Beetles with EAB

Jeffrey Hahn, Extension Entomologist



Jeff Hahn, Univ. of MN Extension

Photo 1: Six-spotted tiger beetle, a common insect in May and June.

The six-spotted tiger beetle is a native insect found throughout most of Minnesota commonly in deciduous forests, along the edges of woods, and in adjacent fields. It can even be found in urban areas in yards and gardens. Watch for this beetle at or near the ground in areas where the sun shines.

This beetle measures about ½ inch in length. It really stands out because it is an iridescent green or blue-green. It also has six white spots, although that number can vary. The six-spotted tiger beetle has conspicuous sickle-shaped mandibles (jaws) and large bulging eyes on the side of its head.

The six-spotted tiger beetle is present in Minnesota from May into early July. It is very active, moving rapidly in short bursts. It is common to see it run rapidly or fly a short distance. As one might suspect from the large eyes and the powerful jaws, this insect is a predator on all types of insects.



Jeff Hahn, Univ. of MN Ext

Photo 1: Emerald ash borer, a slimmer and slower insect than a six-spotted tiger beetle

Besides being colorful and a fun insect to observe, a six-spotted tiger beetle is also important because people may mistake it for an emerald ash borer (EAB), a destructive invasive insect that infests and kills all species of ash. They are both a similar size and bright, iridescent green.

These two insects can be distinguished by the shape of their bodies; EAB is slender, gradually tapering to the tip of its abdomen while the wing covers of a six-spotted tiger beetle are wider than its head. Also a six-spotted tiger beetle is much faster than an EAB.  See also EAB look-a-likes.  If there is any doubt whether an insect is an EAB, capture it and take a picture and submit it to "The Arrest the Pest Line", Arrest.the.Pest@state.mn.us

Monday, June 9, 2014

Damaged Lawns: Steps to Bringing Some Life Back

Sam Bauer, UMN Extension- Turfgrass Science

Spring Damage
The grass-growing season is in full swing, and for some of you this means repairing turfgrass areas that were impacted by winter injury. By now, it should be apparent which areas of your lawn were damaged (but not killed) from winter stresses and which areas will not recover from winter injury. Plants that are slowly recovering, suffered damage only to the leaves and are able to produce new leaves during the spring. Practices such as removing dead leaf tissue and fertilizing will help expedite the recovery of these areas. In contrast, plants that are dead suffered damage to the crown tissue (survival organ of turfgrasses) and will need to be renovated and repaired. The goal of this post is to provide you with information on the different types of winter stresses that effect turfgrass plants and the cultural practices that can be used to minimize winter injury. In addition, a step-by-step outline of the recovery/renovation process is provided.

Repairing dead turfgrass on a yearly basis can be both time and labor intensive and is an unnecessary added expense. Therefore, one of the first steps to minimizing winter injury is to identify the primary cause of damage. In Minnesota, damage detected in the spring may be attributed to several different stresses that the turfgrass is exposed to during winter months. Specifically, there are five main stresses associated with low temperatures and each has the potential to cause damage and/or death of your lawn. Crown hydration is associated with elevated temperatures (above freezing) and results in an increase in water content of the turfgrass plant. This can be lethal if hydrated tissues are then re-exposed to freezing temperatures causing ice crystals to rupture cells in the leaves and crown. Desiccation causes severe dehydration of plant tissues due to lack of snow cover or inadequate moisture and is generally a problem on elevated areas exposed to wind. Prolonged ice cover can also be damaging to lawns by creating an impermeable layer above the turf resulting in a depletion of oxygen and a build up of gasses that are toxic to lawn grasses. Additionally, grasses can die simply from exposure to low temperatures; however, damage associated with temperatures at or below freezing is minor during winters with adequate snow cover. Finally, snow molds are a common occurrence in Minnesota and winter damage associated with these diseases occur every year. For more information on snow molds, visit this article by Michelle Grabowski. Altogether, these stresses can occur individually or as a complex to cause damage that potentially could be lethal to the turfgrass in your lawn.

Along with the five mechanisms causing winter injury to lawns, there are also many other abiotic stresses that occur throughout the spring period. Salt loading from the use of de-icing salts commonly causes damage to turf along roadsides, sidewalks, and driveways. Primarily, these salts cause severe desiccation of leaf and crown tissue and ultimately result in death of the turfgrass plant. By the time the salts are leached through the soil profile with spring rains, most of the damage has been done. In addition to deicing salts, dog urine spots can kill grass from the high salt content and can cause excessive growth due to nitrogen in the urine. Mechanical damage caused by snowplows, mowing too early, and power raking early in the season can also result in areas of turfgrass that need to be repaired in the spring.

Preventing/minimizing winter injury is a yearlong process and involves knowing your lawn and carefully considering the maintenance practices utilized to maintain the turfgrass. For example, in areas that frequently accumulate standing water, core aeration will help with water infiltration as snow melts in response to warming temperatures. Overall, this will reduce the potential for crown hydration and ice crystal formation along with helping to prevent the establishment of an impermeable ice layer. An additional consideration is fertility, specifically associated with the application of nitrogen. Snow molds are more common on lush, succulent turf, and a heavy application of nitrogen in the fall could promote damage caused by snow molds. In addition, succulent turf is more prone to injury attributed to exposure to temperatures at or below freezing. Keep in mind that as summer transitions into fall keep the cultural practices implemented have a direct impact on the survival of turfgrass plants throughout the winter and into spring.

Steps for Recovery
Providing the right conditions for your lawn to thrive is the most important component of a good recovery program. While it's up to Mother Nature to supply the main ingredients, maintenance practices should focus on creating the optimum growing environment for the turfgrass species in your lawn. The following steps outline the processes required for repairing damaged/dead areas of your lawn and also cover factors to consider for managing the overall health of turfgrasses.

1. Choose a mixture of grass to be planted. This is also where the choice of seeding or sodding comes into play. Sod is good for situations where you desire instant turf cover and quick stabilization. With sod, your species and variety options will be limited because not all grasses form an acceptable sod. Kentucky bluegrass is the standard for sod in Minnesota due to its high aesthetic quality and extensive rhizomes that aid in holding the sod together. Recently, fine fescues have been included in specific sod mixtures for the use on roadsides because of their good performance in high salt environments. This sod also makes a great low maintenance option for home lawns.

Sod must be watered daily (more frequently in heat and drought) for the initial week, in the absence of rain. Watering should be focused on wetting the sod and the underlying soil; however, after roots emerge from the sod, irrigation should be less frequent in order to encourage further root growth. Seed should generally be watered multiple times a day lightly. The trick here is to keep the surface moist during the germination period. Excessively wet conditions will cause deterioration of seed and seedlings, and encourage turf diseases and weeds. Remember to avoid watering when precipitation is sufficient.

For help finding the right grass species and seed, visit these resources:

Turfgrasses for Minnesota lawns

Finding the right grass seed

Purchasing turfgrass seed

2. Prepare the area for seeding or sodding. No matter which method of establishment you've chosen, preparation of the surface will generally be the same. The surface should be smooth, weed-free, and not compacted. If seeding into existing grasses, a slit-seeder or vertical mower can be beneficial to ensure good seed to soil contact, but be sure not to plant the seeds too deep (1/4" would be the maximum depth to plant seed). In addition, aeration followed by seeding can also be very successful. For sod, removing existing vegetation and smoothing the surface should prepare areas. The thickness of sod is generally around 1.5 to 2" and this should be accounted for when preparing an area to be sodded. In addition, soil tests can be conducted at this time to determine nutrient status and unfavorable conditions in your soil. Soil samples can be submitted to the U of M Soil Testing Laboratory

3. Apply fertilizer and/or soil amendments as determined by your soil test. If you don't have a soil test, a general recommendation for establishment of seed or sod is to apply a starter fertilizer (high phosphorus, ex. 10-20-10) at a rate of 1lb phosphorus per 1000ft.sq. If the fertilizer in the example is chosen, this would also supply 0.5lb of both nitrogen and potassium. If applying fertilizer only, you have the option to put it down before or after seeding, or above or below sod.

4. Plant seeding or install sod. For seed, be sure to check the seed label for the proper rate. Kentucky bluegrass should be seeded at 1.5 to 2lb per 1000ft.sq, whereas fine fescues should be seeded at 4 to 5lb. Seed can be applied by hand, or preferably with a drop-type spreader. Rake the seed lightly into the soil surface. If sodding, take care to tightly pull the sod seams together. The seams of sod rolls should be staggered in a brick like pattern to avoid channels for water movement. And sod should be installed perpendicular to slopes.

5. TLC. This last step is one of the most important for successful establishment of your new grass. Particularly, moisture content of the surface and soil will be a major determining factor on the recovery rate of those damaged or dead areas. The amount of water necessary for turfgrass establishment varies greatly and depends on factors such as soil type, air temperature, and whether the area has been seeded or sodded. Consequently, monitoring the newly renovated area(s) is key to ensuring that irrigation isn't being over or under applied.

We see sod die and seedlings lost from both over- and under-watering. Unfortunately, there's no general formula for success from a watering standpoint. For seed, the surface should be maintained moist like a sponge. During hot and dry periods this might mean watering 3-4 times per day with 0.05 to 0.10" each time. As seedlings emerge, the amount and frequency of watering can be reduced and this typically occurs approximately 1 to 2 weeks after seeding. For sod, irrigation should be frequent during the first couple of days; however, this should be reduced in order to encourage rooting into the underlying soil. Sodded lawns will benefit greatly from several core aerations in the initial years.

A follow-up fertilizer application can be applied around 2 weeks after seeding to encourage establishment and density. Sod can be mown rather quickly, possibly even a week after being installed if using a hand-operated mower. Larger mowers can damage sod if they are used too soon; before operating large equipment on sod, check to be sure the sod is rooted into the soil. Seeded areas can be cut just as the grass starts to grow beyond the desired height of cut, generally 2.5 to 3" for lawns. Getting seeded areas mown soon will help to reduce weeds and encourage density in the turf, be sure to use lightweight equipment.

By using these five steps you should be able to recover even some of the worst lawn situations. Remember, choose the right plant for the right place and maintain balanced moisture. These are the most important factors throughout the recovery process.


Sunday, June 1, 2014

June 1st Issue of Yard and Garden News

In this issue:

Pests

Insects on the Loose: What's in Your Garden?

European Pine Sawflies are Active Now

Use caution when reading new bee and pesticide research

Diseases

Phomopsis Spruce Decline Found in Minnesota

Gooey Orange Fungi on Junipers

Your Landscape

Extension publications: New & Revised

Where's the Redbud Bloom?

Your Fruit Garden

Andrenid Bees Are Active Now

Andrenid Bees - Critical Pollinators of Apples, Blueberries, & Raspberries

Hardy Kiwi for the Home Garden

Honey bees enjoying a drink on a hot day

Andrenid Bees - Critical Pollinators of Apples, Blueberries, & Raspberries

Karl Foord, Extension Educator - Horticulture





Karl Foord

Photo 1: Andrenid Bee on Willow Flower



Karl Foord

Photo 2: Andrenid Bee on Prunus flower



Karl Foord

Photo 3: Andrenid Bee on apple flower



Karl Foord

Photo 4: Andrenid Bee and blueberry flowers



Karl Foord

Photo 5: Andrenid bees on raspberry flowers

Andrenid bees are one of the earliest emerging bees in the spring. You can see them on willow flowers depending on the type of spring (Photo 1), and we had such a spring this year. Following willows the Andrenids will often be found on Prunus species (plums & cherries) (Photo 2). The next trees and shrubs to flower are apples (Photo 3), blueberries (Photo 4), and raspberries (Photo 5). Andrenid bees are important native pollinators of these species. The next time you put blueberries on your breakfast cereal or make raspberry jam, remember that Andrenid bees have played a significant role in the creation of those fruits.

Honey bees enjoying a drink on a hot day

Karl Foord, Extension Educator - Horticulture




Karl Foord

Photo 1: Honey bee watering hole



Karl Foord

Photo 2: Honey bees hovering around watering hole



Karl Foord

Photo 3: Honey bees hovering around watering hole



Karl Foord

Photo 4: Honey bee hovering while drinking



Karl Foord

Photo 5: Honey bee drinking on edge of puddle

If you think back to last Friday May 30 2014, it was a hot day with temperatures in the high 80's. I was observing pollinators in the apple trees at Pine Tree Apple Orchard. They were draining a low area of one of the fields and in the process created a puddle (Photo 1) just across the field road from a number of honeybee hives. This was a perfect opportunity for the bees to gather around the watering hole, if you will. The bees would hover around the hole looking for a good place to land without getting wet (Photos 2 & 3). One honeybee hovered while she drank (Photo 4), but all the others landed prior to drinking (Photo 5).

Where's the Redbud Bloom?

Over the past 30 years, Minnesotans have enjoyed many mild winters. But the winter of 2013-2014 was a return to the winters of yore. In many parts of the state, the past winter was the coldest in either 35 or 78 years and it is a winter that will be remembered for long persistent periods of very cold temperatures. The persistent cold allowed deeper than normal frost penetration in soils even though snowfall was heavy and just as persistent as the cold temperatures. No matter what statistics you look at - lowest temperatures recorded in the state, average monthly temperature, number of days Minnesota reported the coldest temperature in the nation, amount and persistence of snow cover, soil frost depth, windchill conditions, number of nights with 0 degrees or lower - they all add up to one long, cold, snowy, difficult-to-live-through winter.

The past winter also took its toll on trees and shrubs. Winter burn on evergreens and salt damage on roadside white pines were severe. With the arrival of spring, cold injury to plants that are marginally hardy in Minnesota showed up. Vegetative damage can be seen in most of the repeat-blooming shrub roses whose canes died back to the ground. These plants are now busy sending up new canes from their crowns. Because repeat-blooming roses produce flower buds on this year's canes, these plants will still be able to bloom throughout the summer.



K. Zuzek, UMN Extension

Photo 1: Eastern redbud flower





D. Hansen, MN Ag. Exp. Stn.

Photo 2: Eastern redbud without flower bud winter injury



Winter injury to flower buds occurred among other plant species. Among marginally hardy plants that have been introduced to Minnesota from warmer climates, flower buds are often less hardy than vegetative parts such as leaf buds and stems. An example of this type of injury from the past winter is being seen on eastern redbud (Cercis canadensis). Eastern redbuds are small trees with lavender pink pea-like flowers that open to cover tree canopies in May before heart-shaped leaves expand. Flower buds formed during the growing season of 2013 should have provided us with beautiful bloom this spring. Some redbuds in the heart of the Minneapolis/St. Paul did bloom. But thanks to our low winter temperatures the more common scenario, especially in suburbs surrounding the Twin Cities and in colder outstate locations, was flower bud mortality and lack of bloom.



K. Zuzek, UMN Extension

Photo 3: Eastern redbud with flower bud winter injury



Redbuds are not native to Minnesota. Their native range extends throughout much of the eastern half of North America. In the Midwest, the range extends only as far north as southern portions of Iowa, Wisconsin, and Nebraska. The species is considered hardy to Zone 5 where average minimum temperatures fall between -10 degrees F and -20 degrees F, meaning it lacks the cold hardiness needed to survive and perform well in almost all of Minnesota. But decades ago a large number of redbud seedlings were planted at the University of Minnesota Landscape Arboretum and Horticultural Research Center. Some of these proved hardy enough for use in Minnesota landscapes. Over the last 20 years, seed from these trees have been collected each year and plants grown from this seed are sold as the Minnesota strain of redbud. Even with the improved hardiness of the Minnesota strain, redbud bloom is not 100% reliable and in the most severe of winters (such as the winter of 2013-14) flower buds are killed by low winter temperatures. Cold injury to eastern redbud flower buds used to be more common so that lack of bloom occurred every 4 or 5 years. With winter temperatures trending warmer over the last several decades, redbud bloom has been so much more consistent that this year's lack of bloom may seem unusual to all but the oldest few generations of Minnesota gardeners.


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