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Extension > Yard and Garden News > July 2016

Friday, July 29, 2016

Ground-nesting solitary wasps

Jeff Hahn, Extension Entomologist

There have been a lot of questions about solitary wasps lately. The most common questions have been about cicada killers (Sphecius speciosus) but residents have also seen great golden digger wasps (Sphex ichneumoneus), steel-blue cricket hunters (Chlorion aerarium), and sand wasps (Bembicini).
Cicada killer returning to its nest with a paralyzed cicada. 
Photo: Jeff Hahn, UMN Extension

These wasps are generally large insects. Cicada killers range in size from 1 – 1 ½ inches with a stout body, black and reddish brown thorax, amber colored wings, reddish brown legs, and a black abdomen with yellow bands. Great golden digger wasps are about one inch in length, a more slender body with a black head and thorax covered with short golden hair with a reddish-orange and black abdomen and reddish-orange legs. They have smoky, dark colored wings. Steel-blue cricket hunters are also about one inch in size and relatively slender with iridescent dark blue bodies and wings. Sand wasps are smaller, most are close to ½ inch in length and are typically black and yellow.

Great golden digger wasp with a captured katydid.
Photo: Jeff Hahn, UMN Extension
Their biology is similar to each other. They nest individually in burrows but are gregarious, i.e. there are many nests in a small area. They hunt specific insect prey, paralyze them, and bring them back to their nest to feed to their young. Cicada killers seek out cicadas; great golden digger wasps hunt katydids, steel-blue cricket hunters capture field crickets and sand wasps, depending on the species, look for a variety of different insects, especially flies.

These wasps can be common in lawns, gardens, areas adjacent to sidewalks and driveways, and areas with patio stones. Typically they are found nesting in well drained, light textured or sandy soils. The females are not aggressive and while they are capable of stinging will only do so to defend themselves. Males can aggressively guard a territory and will challenge other males (even people) but they lack a stinger cannot harm us.
Aggregation of dozens of great golden digger wasp nests (with
dozens more to the right).  Photo: Jeff Hahn, UMN Extension

They do not injure turf, nesting in areas where the lawn has thinned or existing bare spots. There can be a large quantity of dirt that is dug up which can be unsightly. It is possible that their tunneling can undermine patio bricks but nothing worse than that. Some people are frightened by them because of their size and perceived threat.

There are a couple of options for dealing with ground nesting solitary wasps. The first is to ignore them. They are not causing any real harm and are not dangerous to people. They will go away eventually on their own sometime during August. However, if they were present this year, they will probably nest in the same area next year.

Sand wasp with paralyzed leaf-footed bug prey.
Photo: Jeff Hahn, UMN Extension
If control is desired, it will be necessary to treat the nests with an insecticide. Generally spraying the nests is not very effective. Instead, apply an insecticide directly into each individual nest entrance. Dusts are most effective; granules and sprays can also help reduce numbers. Effective active ingredients include permethrin and carbaryl. If you rather, you can also contact a lawn service to treat the cicada killers for you.

Resistance does not equal immunity

M. Grabowski, UMN Extension
Crabapple 'Adams' has good resistance to apple scab but has
some leaf spots this year. M. Grabowski, UMN Extension

Planting disease resistant plants is a great way to reduce disease problems in the garden without pesticides or the added time and labor needed for many cultural control practices. A disease resistant plant is able to defend itself against a plant pathogen. In some cases, a resistant plant is very successful in its defense and the gardener will not see any visible symptoms of disease. In other cases the plant may develop low levels of disease but is able to slow the pathogen and prevent severe damage from disease. These plants may be marketed as disease tolerant, moderately resistant, or resistant.

This year, weather conditions have been highly favorable for apple scab, a fungal disease of apple and crabapple trees. The apple scab fungus infects both leaves and fruit. Leaves have olive gray to black spots with a feathery undefined margin. Severely infected leaves turn yellow and fall off mid-summer. This year, due to weather conditions favorable for apple scab, many susceptible varieties of apple and crabapple have thin see through canopies with small piles infected leaf debris collecting on the ground below.

Severe apple scab leaf infection on a
crabapple variety susceptible to apple scab.
M. Grabowski, UMN Extension 
Gardeners may be surprised to see that even apple and crabapple varieties that are resistant to apple scab have some leaf spots and even some leaves turning yellow. It is important to understand that these disease resistant trees have not lost their resistance to apple scab; they are simply fighting against a strong and thriving pathogen population this year. Despite the presence of a few leaf spots, apple scab resistant varieties have thick full green canopies and very little leaf loss. This means that the stress placed on the apple scab resistant varieties is significantly less than the stress on the defoliated susceptible varieties.

Full canopy on crabapple variety ' Adams' despite some apple scab.      
M. Grabowski, UMN Extension 

Apple scab susceptible crab apple variety with
a thin see through canopy due to leaf loss.
M. Grabowski, UMN Extension

For a list of apple scab resistant crabapple varieties and more information about management of apple scab read the UMN Extension publication on apple scab

Clubroot of cabbage

Clubroot symptoms on mustard plants.
M. Grabowski, UMN Extension
New sightings of an old foe

M. Grabowski, UMN Extension

Club root is an unusual disease that results in swollen distorted roots of plants in the cabbage family. Although a few reports from the 1950’ and 60’s indicate that clubroot occurred in Minnesota at one time, this disease has not been common in recent years. To determine just how wide spread this disease is in Minnesota, the Minnesota Department of Agriculture has been looking for clubroot as part of their pathways survey for new and emerging invasive plant pests. The disease was identified in Ramsey county on edible mustard greens in the 2016 survey. 

Firm white galls from clubroot infection of a turnip root.
G. Holmes, CA Polytech State University
What is clubroot?
Club root is a plant disease caused by Plasmodiophora brassicae, an interesting microorganism that can only grow and reproduce within the roots of an infected plant. The clubroot pathogen infects members of the cabbage family including broccoli, cauliflower, radish, turnip, Brussels sprouts, rutabaga, and mustard greens. Although the pathogen can infect and survive on the roots of some grasses and weeds, only members of the cabbage family develop swollen distorted roots.

How to recognize a plant with clubroot
Cabbage plant wilting because of clubroot.
G. Holmes, CA Polytech State University
Plants infected with clubroot may be stunted and have small leaves. Because the roots are not functioning properly, infected plants wilt with only slight drought stress and may turn yellow. Once plants are dug up the infection is quite obvious. Roots are swollen and distorted into large clubs. Smaller bulbous galls may be seen on root branches off the side of larger clubbed roots, or coming off a large taproot like a turnip. Infected roots are white and firm early in the growing season but turn dark and begin to decay by the end of the season. As these galls decay, resting spores of the pathogen are released into the soil. These thick walled spores are capable of surviving in the soil for over 20 years.

Managing clubroot in the vegetable garden
Although the clubroot pathogen is very difficult to get rid of once it is found on a site, there are several strategies a gardener can use to reduce damage from this disease.

  • Bury infected plants at the same site where the disease was discovered. Moving infected plant material to a compost pile or a different part of the garden will spread the pathogen to new areas.
  • Do not plant any members of the cabbage family in the garden for 5 to 6 years. This will not eliminate the pathogen but will reduce it to a less damaging level. Consider planting members of the cabbage family in pots, in a community garden plot, or in a flower bed to allow the infested area of the garden to remain free of susceptible plants for the full rotation period.
  • Have a soil test done. If the soil pH is lower than 7.0 add lime to the soil to increase the pH above 7.0. This will not kill the pathogen but will create conditions that are unfavorable for disease.
  • Clean garden tools and equipment before using them in other areas of the garden. Do not move soil from the infected area of the garden, as the soil will contain clubroot spores.
  • Look for varieties that have resistance to clubroot. Several cabbage varieties with resistance to clubroot are available. Check seed catalogs for resistant varieties of other members of the cabbage family.

Thursday, July 28, 2016

Earwigs are temporary nuisance

Jeffrey Hahn, Extension Entomologist
Adult earwig.  Note the pincers on the tip of the abdomen.
Photo: Jeff Hahn, UMN Extension

Some people across the state have been encountering earwigs on their property this month. While most people recognized these insects, some were not sure what they had found. Earwigs are distinctive looking. They are about 5/8 inch long, beetle-like, with a flat, reddish brown body and very short wings. The feature that makes them easy to identify is the conspicuous pinchers (cerci) on the tip of their abdomen. 

Most questions deal with earwigs as nuisance invaders into homes and other buildings (some people also have problems with them in their gardens damaging plants).  Earwigs can enter buildings, sometimes in large numbers. Although they do not cause any actual damage, they are annoying and people want to control them. If you are having problems with earwigs in your homes, try these steps to minimize them.

You can reduce the number that is outside by using traps in the landscape. This could be with rolled up newspapers, cardboard tubes, or similar objects set up outside where you find earwigs. They will crawl inside these objects by early morning in order to hide. You can then shake them into a pail of soapy water to dispose of them.

Check around the outside of your home and seal or repair any openings or gaps you find that allows earwigs to get inside. Check particularly around the foundation, windows and doors. Also examine where the siding and foundation meet as well as the areas around water faucets and vents. If you are finding large numbers, you can supplement this with an insecticide application around the exterior the home, e.g. permethrin.

Once they get inside, the only practical control is physical removal.  Vacuum them or sweep them up.  The good news is that they do not reproduce indoors and will eventually go away on their own sometime during August.

For more information, see European earwigs in homes and gardens.

MSHS hosts Will Steger to speak on global warming

Eyewitness to Global Warming
Thursday October 13, 7 to 8:30 p.m.
McNeely Hall, University of St. Thomas
2060 Summit Ave, St Paul.
$20 MN State Horticulture Society members, $25 nonmembers
Pre-registration required: Tel: 651-643-3601 or 800-676-6747

Will Steger, world renowned polar explorer, educator, photographer, writer and lecturer will present a retrospective of a life in the arctic regions of the world. He has logged thousands of miles of travel by dogsled and has become a voice calling for understanding and the preservation of the arctic. Join us for his vivid account of the changes that he’s witnessed firsthand, caused by global warming pollutants, in Arctic regions over four decades of polar exploration. Steger shares stunning photographs from his expeditions along with compelling data and satellite imagery to document the deterioration in the polar ice caps.

While the issue is critical, and the presentation is dramatic, Steger’s message is one of hope and empowerment. An understanding of our role in the causes and effects of global warming make this personal. But as Steger explains, solutions are readily available and by making economically and environmentally smart choices people can make a difference, and specifically what we as gardeners can do to have a positive impact.

This event is co-sponsored by the Minnesota State Horticultural Society and the University St. Thomas. Enrollment is limited, and pre-registration is required. Refunds will not be issued for nonattendance except by cancellation at least one week prior to the event.

Friday, July 22, 2016

What to do about Japanese beetles

Jeffrey Hahn, Extension Entomologist

Japanese beetles have taken a lot of people by surprise by their abundance this summer, especially in the Twin Cities. They were numerous in the early 2010’s but a series of very dry summers reduced their populations. Japanese beetle eggs and recently hatched grubs do not survive very well in dry soils. However, we have had more normal precipitation the last few years which has allowed Japanese beetle numbers to rebound. Japanese beetles also started emerging sooner than expected this year as they were first sighted in late June. As a consequence, a lot of people are faced with damaged plants and questions about what they should do now.

One common question people ask is whether their tree will die because of Japanese beetle feeding. If the tree is healthy and mature, the tree is not going to die in one year from Japanese beetle feeding. Trees are quite resilient and can tolerate a lot of defoliation.

It is actually easy to kill a Japanese beetle but it is when they are so numerous that it is challenging to manage them so their damage is at acceptable levels. Physical removal is a good option, although it is a labor intensive method; plus if you are trying to protect a tree you will only be able to reach so high to get them. However, this is the best non-chemical option.
Although this linden does not appear to be to
be healthy, it will not be killed by Japanese
beetle feeding and will recover.  Photo:
Jeff Hahn, UMN Extension

Another non-chemical method that people consider using is Japanese beetle traps. While they can capture an impressive number of beetles, it is not likely to protect your garden. In fact, research has shown that when these traps are used in home gardens, you are more likely to see more damage as the traps attract more Japanese beetles than they can capture.

There are a few low impact insecticides available. Pyrethrins are effective but this product has no residual and beetles have to be hit directly by the spray. It also needs to be reapplied more than once. Neem oil is also an option. It helps to deter Japanese beetles but is less effective when large numbers are present. Both of these products can be toxic to pollinators so be sure to apply them when bees are not active.

There are residual insecticides that you can use, like permethrin or carbaryl (Sevin). To protect bees, you need to apply insecticides during late evening after bees are not active. The products should dry by morning when the bees become active again. If you are trying to protect a large tree, you may need to have it treated by a landscape or tree care company.

Deciding whether to treat trees and other plants will depend on how much of the leaves is still intact. If nearly all of it is already chewed up, then spraying does not help them. If there are still a lot leaves that are not affected yet, then it is worth your while to go ahead treat.

Some people wonder whether they should treat the grubs in their lawns to reduce the adults. However, this is not effective. The adults are very mobile and can easily fly onto your property from adjacent areas. Only treat your turf if you are seeing turf damage. The best time to treat preventatively is about the time Japanese beetles are laying eggs which late June or early July. You can treat curatively into August. There are a variety of products that can be used. Insecticides containing chlorantraniliprole (e.g. GrubEx) can be used both curatively and preventatively.

Friday, July 15, 2016

Dealing with Wind-Damaged Trees

Kathy Zuzek, Extension Educator

High straight-line winds and tornadoes have been all too common in Minnesota in June and July. If trees on your property were damaged during these storms, you may be wondering whether to salvage them and how to salvage them. Here are some considerations.

Safety always comes first!
  • As you start to assess tree damage, check for any downed power lines. Stay away from any power lines and call 911. 
  • Check tree canopies for hanging large limbs that may drop to cause injury. If they are present, call a professional arborist

Assess the damage to determine if your tree should be salvaged. Many factors play into this assessment:

  • Health of the tree prior to wind damage: If previous issues – unattractive habit, poor health from diseases, decay, insects, soil problems, salt damage, etc. – were already present before the wind damage, you may want to remove and replace your tree. 
  • Windthrown trees with roots ripped out of the ground: This usually occurs on larger trees because of some already-present root defect. Larger trees are not salvageable. Smaller trees less than 25’ in height may be salvageable. Uprooted trees should be straightened, replanted and staked immediately after the storm so that root desiccation is avoided. More information on staking can be found here
  • Leaning trees are hazardous and should be removed.
  • Trees with major trunk failure, such as large noticeable cracks and snapped off canopies should be removed. 
  • Canopy condition: Were large structural limbs broken off? How much of the canopy is left? If more than 50% of the canopy was damaged, the tree should be removed. 
  • How will the tree look in the landscape after pruning to correct wind damage? Is the tree too damaged for it to be an asset in your landscape? If so, consider removing and replacing the tree. 
  • If you are unsure about retaining or removing your tree, seek the advice of a professional arborist.
Determine how much of the corrective work you are capable of doing: 
    Never prune through a branch collar
     or a bark ridge. Graphic credit:  U of M
  • Consider safety factors again! Ask yourself if you are experienced enough to handle all of the pruning work ahead of you. If corrective measures require a chainsaw, removal of large overhead branches, working off the ground and in the tree canopy, or removal of trees or branches near electrical lines or structures, consider hiring a professional arborist with the expertise to do this work safely. Also check your tree for bent and twisted branches that may be under pressure. Removal of these branches can be dangerous as a sudden release of pressure can cause a sudden and unpredictable kick back. 
Use proper pruning techniques:

The last thing you want to do is create more problems for your damaged tree. Poor pruning cuts serve as entry points for decay: Jagged storm wounds on branches are slow to heal and allow decay to enter. Remove these wounds with a clean pruning cut.
Don't leave branch stubs.
Photo credit:  K. Zuzek
  • Never cut through a branch collar (the swollen base found on some branches) or a bark ridge (a horseshoe-shaped raised bark ridge formed at a branch and trunk union and extending down both sides of the branch/trunk juncture) during branch removal. 
  • Never leave long branch stubs. 
  • Damaged branches that are long and/or heavy should be removed with a 3-step removal cut to prevent bark stripping below the cut. 
  • Wound dressings should not be applied as they usually do more harm than good. There is one exception. Wound dressing applications should be made to fresh pruning cuts on oaks if pruning occurs from April through mid-July in areas where oak wilt is a problem. This deters the insects that spread oak wilt when they feed on fresh wounds such as pruning cuts.
3-step removal.  Graphic credit:  U of M Extension
Bark stripping.  Photo credit:  K. Zuzek

Thursday, July 7, 2016

From the DNR: Advice for dealing with storm damaged trees

Reprinted from the MN DNR - July 6, 2016

Cleanup following a storm can be an overwhelming task for homeowners. Knowing which trees to save and which to remove can impact safety and the survival of remaining trees, according to the Minnesota Department of Natural Resources.

DNR forestry outreach specialist Jennifer Teegarden offers the following tips.


  • Approach damaged trees with caution. Stay clear of downed wires and call 911.
  • Carefully inspect standing trees for damage and deal with hazardous trees first. If possible, ask a forester or arborist for advice.
  • Trees should be removed if more than 50 percent of the trunk or live branches in the crown are damaged, and if the tree is unnaturally leaning or roots are damaged.
  • Watch for detached branches, loosely hanging branches and split or cracked trunks that can cause injury or further damage.
  • Use proper pruning techniques to remove broken limbs by cutting just outside the branch collar, but limit pruning to making the tree safe. Too much pruning can weaken an already stressed tree.
  • Water stressed and damaged trees weekly to help them repair and rebuild. Be careful not to overwater, especially in heavy clay soils.
  • Monitor damaged trees in upcoming years to make sure they don’t become a hazard.
  • Be rushed by promises of bargains from inexperienced or unqualified tree service providers. Improper pruning or unneeded removal can result in unnecessary costs or loss of healthy trees. Ask for references and proof of insurance.
  • Repair a broken branch or fork of a tree with tape, wire, bolts or other wraps. It will not heal, and the split will invite decay and further weaken the tree. Cabling or bracing should only be performed by a certified arborist and inspected annually.
  • Remove the tops of trees. This makes the tree more susceptible to insects and disease, and results in new branches that are weakly attached.
  • Apply paint or dressing to wounds as these materials interfere with the natural wound sealing process.
  • Remove small, leaning trees. Trees less than 15 feet tall may survive if they are gently pulled back into place. Press out air spaces in the loosened soil. The tree can then be staked for up to a year.
  • Fertilize stressed or damaged trees.
Information on tree care, proper pruning techniques and handling damaged trees is available on the DNR website at

For more extensive information on tree care, contact a DNR forester, city forester, certified arborist or county Extension staff.

Take the right steps to correct damaged trees so they can continue to provide shade, clean air, beauty and increased property value for many years to come.

Friday, July 1, 2016

Prevent blossom end rot

M. Grabowski,  UMN Extension

The current warm, dry weather combined with fast growing tomato plants creates ideal conditions for blossom end rot, a common problem in garden tomatoes. Fruit affected by blossom end rot have a tan to black, flat, leathery area on the bottom (blossom end) of the fruit. 

Fungal spores may be visible on the discolored area and rot may extend into the fruit, but this problem is not caused by a pathogen.  Blossom end rot is the result of a calcium deficiency in the growing tomato fruit. Any bacteria or fungi present are secondary organisms, taking advantage of the weakened fruit.

Although blossom end rot is caused by a calcium deficiency in the fruit, it does not mean that there is a lack of calcium in the soil. Often blossom end rot occurs as a result of several cultural or environmental factors that affect the plant’s ability to take up calcium. Fluctuations in soil moisture, heavy applications of nitrogen fertilizer, and injury to roots can all predispose tomato plants to blossom end rot.

Although symptoms may not yet be visible, now is the time for gardeners to take steps to prevent blossom end rot.

  1. Keep the soil around the tomato plant evenly moist but not waterlogged.  Do not allow the soil to completely dry out between watering. Fast growing tomato plants use lots of water on a hot, sunny day. Mulch the soil with straw, wood chips, or plastic mulch to reduce moisture loss. Touch the soil to determine if it needs water. Soil type, exposure to sun and wind, and plant size will all affect how frequently a tomato plant will need to be watered. Potted tomato plants may need to be watered more than once a day. 
  2. Do not over apply fertilizer to tomato plants with young growing fruit. Excess fertilizer results in quick growth spurts that can increase blossom end rot. In addition, many of the nutrients in fertilizers compete with calcium for uptake by the roots. Tomato plants do not need fertilizer with phosphate or potash at this time, but can be fertilized with 0.15 lb/100 ft2 of nitrogen if needed. Garden fertilizers will list three numbers on the label. These numbers represent that amount of nitrogen – phosphate –potash in the fertilizer. A fertilizer containing only nitrogen will have a number followed by two zeros (16-0-0). Follow the instructions on the package to apply the appropriate amount. 
  3. Avoid injuring tomato roots. Do not dig in the soil within 1 foot of the stem. 
  4. Some varieties are more susceptible to blossom end rot. If severe blossom end rot occurs regardless of good cultural practices, consider using a different variety next year.

Do not give up on the tomato plant if blossom end rot does occur. Often the first fruit are the most severely affected. Fruit produced later in the growing season may not have blossom end rot at all.

Fruit with blossom end rot can be eaten if the rotten part is cut out and the fruit is cooked. Any type of rot can affect the natural acidity of the tomato fruit. As a result, tomato fruit with rot should never be used for canning because the natural acidity plays an important role in keeping out harmful microorganisms. The fruit can be cooked and eaten, cooked and stored, or frozen, cooked and eaten.

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