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Monday, November 2, 2015

What creates "fall color"?

Kathy Zuzek, Extension Educator - Horticulture

During a process called acclimation, trees and shrubs prepare for winter and its low temperatures by ceasing growth and becoming dormant.  This process actually starts in late summer in response to decreasing day length and is rarely noticed by our human eyes.  But from September through November, as day length continues to decrease and temperatures drop, we can’t help but notice and appreciate the process of acclimation as deciduous trees and shrubs burst into vivid hues of red, orange, and yellow. These colors signal that trees and shrubs are reducing their photosynthesis levels and moving nutrients from leaves to stems and roots that serve as storage locations in winter. The variety of fall colors we see are created by the interaction of four pigments – chlorophyll, carotenoids, anthocyanins, and tannins – and variations in temperature, light levels, and water supply.  Each of these pigments is uniquely effective at absorbing particular wavelengths of light.  The foliage colors we see are dependent on the wavelengths not absorbed by pigments which are instead reflected to our eyes.

Absorption spectra of chlorophyll and carotenoids
Leaves are energy factories for plants.  They are packed with organelles called chloroplasts  containing the pigment chlorophyll which absorbs and harvests the energy of sunlight. Through the process of photosynthesis, this light energy is used to convert carbon dioxide and water into energy rich carbohydrates that plants then use and store for their growth processes.  Chlorophyll absorbs violet, blue, red and orange wavelengths very effectively but does a poor job of absorbing green wavelengths. Instead green light is reflected and leaves appear green to our human eyes.  Chlorophyll is a short-lived molecule that is easily degraded by bright sunlight.  As it degrades during the growing season, new chlorophyll is constantly produced until autumn when shortened day length and decreasing temperatures signal plants to slow the production of chlorophyll.  

A sugar maple high in carotenoids

Source:  K. Zuzek, UMN Extension

Carotenoids are accessory pigments found in plants.  Like chlorophyll, they are also bound to membranes in chloroplasts and are present throughout the growing season. Carotenoids are also found in chromoplasts, organelles similar to chloroplasts except that they lack chlorophyll.  They absorb blue and blue-green wavelengths and pass this harvested light energy on to chlorophyll for use in photosynthesis.  Carotenoids are much more stable than chlorophyll and have another important role in plants.  They help to protect against excessive light and by doing this, extend the life of the more fragile chlorophyll pigment.  Chlorophyll and carotenoids working together during the growing season remove violet, blue, red, and orange wavelengths of light, leaving bright green as the reflected color that our eyes see in leaves.  But in autumn when chlorophyll production stops and only carotenoids are left, yellow, orange, red, and some green wavelengths are reflected and the human eye sees the yellow and orange colors of willows, honeylocusts, lindens, birches, aspen, maples, elms, ginkgos, and hickories.

A sugar maple high in anthocyanins

Source:  K. Zuzek, UMN Extension
Anthocyanins are a third group of pigments found in some but not all trees and shrubs.  Contrary to chlorophyll and carotenoids, they are water soluble and are found in vacuoles, which are organelles   Depending on the type of anthocyanin and the pH of the sap in vacuoles, they reflect and appear pink, red, scarlet, and purple in flowers, fruit, and autumn leaves. When anthocyanins and carotenoids are both present in leaves, we are gifted with brilliant orange fall color.   Anthocyanins also absorb light in the ultraviolet spectrum so they serve as a sunscreen to protect plant cells from UV damage.  In most trees and shrubs, anthocyanins are produced in autumn when cool temperatures promote the conversion of starch in leaves to sugars.  These higher sugar levels then react with proteins to form anthocyanins.  As anthocyanin levels increase and chlorophyll production slows in fall, we see brilliant reds and oranges in red, sugar and Amur maples, sumacs, juneberries, and burning bush.  This process is light driven so leaves in full sunlight will be more intensely colored than those growing in shade.  This is why plants growing in full sun will turn brilliant red in fall while plants growing in shade are less brilliant.
that resemble tiny water balloons. Anthocyanins absorb blue, blue-green, and green light.

A fourth group of pigments that impact fall color is the tannins.  Tannins are found in vacuoles and in cell walls and play an important role in plants’ defense mechanisms against pathogens, insect feeding, wildlife browsing, and environmental stresses.   Like carotenoids, they are always present but are only visible when chlorophyll levels decrease.  Tannins are responsible for the brown fall color seen in oaks and other species.  They also mute the brilliance of carotenoids and anthocyanin pigments, resulting in the deep and rich burgundies, copper, and golds we see during autumn on oaks.

The rich tapestry of fall color we enjoy in Minnesota is impacted by genetics, weather, and climatic factors.  Genetics determine whether a tree or shrub can produce only some or all of the four pigments involved in fall color.  Genetic variability between individual plants creates subtle differences in color from tree to tree within any one species. Decreasing day length and temperatures in fall trigger a reduction in chlorophyll production.  As this happens, yellow, orange, and brown fall colors appear as the carotenoids and tannins that are always present are unmasked.  If weather conditions are favorable for anthocyanin production - lots of sunny days to drive the process of anthocyanin production; the absence of high winds or freezing temperature that remove or damage leaves before peak fall color; cooling daytime temperatures that favor sugar production; and cool (but not freezing) night temperatures that slow the movement of sugars out of leaves - anthocyanin-producing plants will provide us with the brilliant orange and red fall colors that result in a good year of fall color. If weather conditions are not so favorable to anthocyanin production – cloudy days, excessively high or low temperatures, early frost, high winds - we have mediocre or poor years of fall color.  

The Minnesota Department of Natural Resources produces an interactive map called the Fall Color Finder that can direct you to regions of the state displaying best fall color. 

Want lemons? Act like a bee

Julie Weisenhorn - Extension educator - Horticulture

Hopefully by now, Minnesota gardeners have brought their precious houseplants indoors for the winter. But what about fruit trees? Some, like the Meyer lemon, require bees to do the pollinating. Once indoors the gardener needs to step in ...


Friday, October 30, 2015

Fun trivia about plants and Halloween

Julie Weisenhorn - Extension Educator - Horticulture

Going to a Halloween party? Here are some fun plant-related trivia to wow your friends ...

Halloween & the seasons
Food and the fall harvest is at the core of our traditional American Halloween. It is a combination of Christian traditions and the ancient Celtic Pagan festival Samhain (pronounced sah-win) which means “end of summer” and the harvest. It was a time to commune, put up resources for winter, and bring animals in from the summer pastures. Times of transition in the natural world were thought to be special and supernatural, creating the belief among some that Samhain was a time when spirits of the dead would cross over to the living world. However, the festival of Samhain as more about changing seasons and preparing for dormancy of nature as summer changes to winter.
Sources: History of Halloween
History of Halloween - Live Science

Pumpkins are a symbol of Halloween. The Latin name for pumpkins is Curcurbita pepo. “Pepo” means “to ripen in the sun”. Squash and zucchini are also in the Curcurbita genus and the three plants sometimes cross-pollinate resulting in some unusual fruits and a baffled gardener. Pumpkins are thought to be some of the oldest domesticated plants on Earth. The first pumpkins were small hard gourds cultivated around in 10,000 B.C. in Mexico. The winner of the 2015 Giant Pumpkin contest - a popular attraction at Minnesota State Fair - weighed in at 1,473 lbs. and was grown by Bill Foss from Buffalo, MN. The record for the largest pumpkin grown in North American was set by Gene McMullen and weighed in at 2,145 lbs. It was shown at 2015 Cedarburg Wine & Harvest Festival, Cedarburg WI.

Pumpkin carving is a tradition based on an Irish myth about an unsavory character named“Stingy Jack”. Stingy Jack's questionable dealings with the Devil during his life led to him wandering through the darkness of purgatory with only a piece of burning coal in a carved turnip to light his way. He became referred to as “Jack of the lantern” or “Jack o’lantern”. Each year at Halloween, the Irish carved turnips or potatoes and placed them in windows to ward off evil spirits including Stingy Jack. Pumpkins were used instead of turnips when the Irish brought the tradition to America. The first image of the modern jack-o-lantern appeared on the cover of Harper’s Weekly in 1867.
Source: All about pumpkins

Apples have been used for making life predictions during Halloween festivities. Bobbing for apples in a tub of water has long been a party game with the goal of grabbing an apple in one's teeth without using your hands. The first to successfully bob for an apple was destined to be the first to marry. Likewise, on Halloween, young women would peel apple in a continuous strip and throw it over their shoulder where it would supposedly land in the shape of the first letter of her future husband’s name. Imagine if that young gentleman also grabbed the first apple from the tub of water!
Source: History of Halloween - Live Science

Witches’ Broom
Witches' broom is a bushy bunch of twiggy, weak stems protruding from tree or shrub. It can be caused by environmental and pathological stresses that lead to formation of the witches' broom by the plant. Environmental stresses can damage growing points and pathogens can cause abnormal growth when they attack a host plant. Depending on the plant, environmental stresses include road salt and  herbicides. Pathogens that cause witches' broom include fungi, mites, aphids, Phytoplasmas and parasitic plants like dwarf mistletoe. Sometimes genetic mutations occur in a plant and create a witches' broom growth. Some larger growths on conifers like Norway spruce have been successfully propagated to produce new dwarf conifer cultivars. In Minnesota, witches' brooms are commonly seen on spruce and caused by the parasitic plant dwarf mistletoe. This plant lives its entire life within the canopy of the tree, stealing nutrients and water from host plant. Large witches' brooms may kill plant over time. Management of witches' brooms includes pruning out offensive branches,  removing plants with over 50% dead branches, and avoid planting spruce or other susceptible conifers near infected trees.
Sources: Witches' brooms sightings in trees
Eastern spruce Dwarf Mistletoe
Witches' broom

The Ghost Pepper (Bhut Jolokia)
The ghost pepper originated in Assam, located in northeast India, and was introduced in the Western world in 2000. Bhut is translated as "ghost" due to fact the heat sneaks up on the consumer. The key agent in peppers - what gives them their "heat" - is called Capsaicin, a neuropeptide releasing agent.   Peppers are rated in scovil heat units (SHU) determined by how much sugar syrup it takes to completely offset the heat of the capsaicin. For example, a green bell pepper rates at zero SHU. On the other end of the spectrum pure capsacin rates at 15 million SHU. The ghost pepper is rated at 1,041,427 SHU - three times the former winner, the Red Savina habanero pepper and certifying the ghost pepper by the Guinness Book of World Records as the hottest chili pepper on Earth. CAUTION: This is a seriously hot pepper and while it is an interesting plant, I am not recommending its consumption. It apparently can cause severe pain and intestinal distress, resulting the medical attention.
Sources: Ghost Pepper

The gut-wrenching science behind the world's hottest peppers 

Update 11-02-15: Just heard about two peppers that rank hotter (!) than the Ghost Pepper on the Scoville scale: Naga Viper (1.38 million) and Carolina Reaper (1.56 million SHU and noted as the hottest pepper in the world).  


Tuesday, October 27, 2015

EAB found in Duluth

 Jeffrey Hahn, Extension Entomologist

(The following information is taken from an October 23, 2015 news release from the Minnesota Department of Agriculture)

The Minnesota Department of Agriculture (MDA) last Friday identified emerald ash borer (EAB) in the city of Duluth (St. Louis County). Finding EAB in Duluth is not a surprise as Superior, Wisconsin, just across the state border from Duluth, confirmed EAB in August, 2013.

MDA staff found EAB larvae in an ash trees on Park Point. The find was discovered as part of a three-year
EAB was found in Duluth by sampling branches by
bark peeling.  Photo - Jeff Hahn, UMN Extension
study the MDA is conducting in partnership with the city. The study is evaluating different methods for finding EAB, and one of those methods is removing samples of branches from ash trees to peel back the bark and look for signs of the insect. MDA staff found evidence of EAB in four of 35 trees sampled in this way.

Because this is the first time that EAB has been identified in St. Louis County, the specimen have been sent to the United States Department of Agriculture (USDA) for confirmation, which is expected within days.

MDA will be implementing an emergency state quarantine of Park Point in Duluth. A quarantine is designed to help prevent EAB from spreading outside of a known infested area by limiting the movement of any items that may harbor EAB, including ash trees and ash tree limbs, as well as all hardwood firewood.

St. Louis County becomes the 12th county in Minnesota to verify EAB. EAB was also confirmed in 2015 for the first time in Scott, Chisago, Fillmore, Anoka, and Washington Counties. This invasive beetle has also been found in Hennepin, Houston, Olmstead, Ramsey, and Winona counties. These counties are all under quarantine.

The biggest risk of spreading EAB comes from people unknowingly moving firewood or other ash products harboring larvae. Take these steps to minimize spreading EAB: 
  • Don’t transport firewood.  Buy firewood locally from approved vendors, and burn it where you buy it. 
  • Be aware of the quarantine restrictions. If you live in a quarantined county, be aware of the restrictions on movement of products such as ash trees, wood chips, and firewood. 
  • Watch your ash trees for infestation. If you think your ash tree is infested, go to and use the “Do I Have Emerald Ash Borer?” guide.
For more information about EAB, see Emerald ash borer in Minnesota. See also the MDA news release, MDA identified emerald ash borer in the city of Duluth.

Friday, October 23, 2015

Q&A: Fall lawn care

Sam Bauer, Extension Educator - Horticulture
Lawn care is a hot topic even as we move into cooler weather! Here are some answers to common questions from WCCO Smart Gardens listeners and Yard & Garden News readers.

Q: What mowing height should I keep my lawn at before winter?
Cool season grasses still
need care in the fall.
A: Generally we suggest to keep your lawn at the same height as you've had in the fall.  Cutting the lawn short prior to winter has been commonly suggested in the past as a means of reducing spring damage from snow molds and voles, but cutting the lawn too short will be more of a stress to the grass than the injury you may experience from diseases or critters.  If the standard mowing height for your lawn is 2.5 to 3 inches, we suggest to keep it at that.  If the height is 3+ inches, then we would recommending bringing it down to 3 inches before winter.  If you do plan to lower the mowing height, be sure to mow several times at this height, not just the final mow.  The goal is to condition the grass to this new height with several mowings prior to winter.  Again, we may only suggest this if you have had snow mold issues in the past of if you maintain your grass at a long height in the fall.  Also, be sure to continue mowing until the grass stops growing, this will help reduce snow molds and winter damage.  You can use a bagging attachment on the last mow of the year to help remove any excess organic matter and leaf litter. 

Q: Should leaves be raked off of the lawn or mulched with a mower?
A: It depends.  For homeowners with numerous trees, it may be impractical to mulch all of them into the lawn without smothering the grass at some point.  In that case we would suggest mulching a majority of the leaves into the lawn and raking up the rest.  Be sure you can see at least 80-90% grass after mulching leaves, this will ensure that the leaves aren't smothering the grass.  To practically mulch all of your tree leaves, you may need to be out with a mower more often than your grass needs to be cut, because if too many leaves fall you may not be able to mulch them into the the lawn.

Tree leaves contain organic matter and many nutrients that can be beneficial to your lawn.  For example, a study conducted in New Jersey on 100 municipal trees demonstrates nutrient content of 1% nitrogen, 0.1% phosphorus, 0.38% potassium, also secondary macro nutrients and micronutrients (1).  The organic matter will also benefit the lawn my increasing moisture holding capacity and improving aeration.  Standard mowers will work, and we suggest to close the side discharge for mowers that have one.  Closing the side discharge will contain leaves in the mower so they get chopped up better before they fall into the grass canopy.  Mulching blades can be purchased as well.
Here are some resources that help to further explain leaf mulching:
WCCO Good Question: Do we really need to rake?
Minnpost: Leaf bagging under scrutiny as a wasteful expense and pointless chore

Q: Is it too late to fertilize my lawn?
A: Yes.  New research on late-fall fertilization demonstrates that a majority of the fertilizer applied in late-fall (late-October or early-November) can be lost to the environment because lawn grasses are not able to absorb fertilizer as well when temperatures are low.  For Twin Cities residents, we suggest to not apply fertilizer past mid-October. Here is more information on this research and our recommendations:
Apply lawn fertilizer by mid-October
Upper Midwest Lawn Care Calendar for Cool Season Grasses

Q: What is dormant seeding and when should that be conducted?
A: Dormant seeding is a practice that involves seeding when temperatures are too low for the seed to germinate prior to winter, and it is expected that the seed will germinate in the spring. This can give you a jump on spring seeding.  Germination prior to winter is bad and seedlings will generally die if they haven’t matured. Sometimes it is a bit of a waiting game at this time of year. The trick is to find the time when soils are unfrozen so that seed can be worked in slightly, yet air temperatures must be cold enough so the seed won’t germinate. Wait for high daytime temperatures of 35-40 degrees before seeding.

Q: Is there an advantage to dormant seeding versus spring seeding? A: Yes and no. A dormant seeded lawn could mature as much as one month faster in the spring than a spring seeded lawn. This is because some of the germination process actually starts prior to winter in a dormant seeded situation, although the shoots still haven’t emerged from the seed. When temperatures are adequate in the spring, complete germination occurs. In this case the seed actually dictates when temperatures are warm enough to grow. Just like late-fall, temperatures and weather patterns can be unpredictable in the spring. For this reason, the best timing for spring seeding is difficult to predict, which can delay the timing to actually sow seed. Still, there some negative aspects of dormant seeding to consider. First, because of the spring temperature fluctuations, it is possible to have good seedling establishment initially, but a cold spell during this time will injure these seedlings. Also, there is a greater potential for seed loss over the winter due to erosion and water movement, predation, and decay. These positive and negative aspects should always be considered during this process. Here is more information on dormant seeding lawns:
Dormant Seeding Lawns: Last chore of the season

Monday, October 19, 2015

Timely questions about fall gardening

Julie Weisenhorn, Extension Educator - Horticulture

A Minnesota fall landscape
Even though the weather is cooling down, days are getting shorter, and plants are moving into dormancy, Minnesota gardeners are still concerned about employing best practices for healthy landscapes. Here are a few common questions that might help you put your garden to bed (and get ready for next year's growing season!)

Keep watering your trees and shrubs too - especially evergreens!

Q: Can I prune my shrubs roses now? How far should I cut them back? 
A: You can cut back your roses now. Remove 1/3 of the plant as a rule of thumb, but of course, if the plant is blocking a window, walkway, etc., then you may cut more back. Here is an Extension publication on caring for roses that includes winter protection for roses. Enclosing roses in fencing stuffed with leaves will protect the plant from winter desiccation and start the next growing season with more live tissue. The fencing can also help protect your roses from browsing animals.

Q: Is it OK to cut down my perennials before they turn brown? Also, is it still OK to transplant them this late in the year? 
A: You should leave perennials alone until they die back (turn brown and dry). As long as they are green, they continue to photosynthesize and build up carbohydrates in their roots for better growth next year. Best too to hold off transplanting till spring. Plants need more time to re-establish their roots after transplanting and it is easier to handle the plants prior to leafing out. Hint: If your perennials have sturdy stems with interesting seed heads (like roses, asters, coneflowers, black-eyed susan, etc.) consider leaving them in place to provide winter interest in your landscape.

Q: Is it too late to seed my lawn? 
A: It is too late for normal seeding. However, you could wait till mid-November and do dormant seeding. This is seed that will germinate in the spring. Here's an article on dormant seeding. Also, here is an excellent calendar for the upper Midwest that takes the mystery out of lawn care. There's still time to treat for broadleaf weeds too - especially creeping charlie. (for managing creeping charlie, look for a product with glyphosate and triclopyr). Always read and follow all instructions on the product label. The label is the law.

Tuesday, October 13, 2015

What's That Fungus?

Have you ever wondered what mushrooms are growing in your yard or wanted to learn more about the fungi you saw on trees, logs or growing through the leaves last time you hiked in the woods? A great resource for those who would like to learn more about the fungi in our world has been created by the University of Minnesota Mycology Club. Free downloadable flash cards contain color photos, scientific and common names, habitat and descriptions of common Minnesota fungi. Check them out today!

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