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March 2014 Archives

Winter Burn on Minnesota's Evergreens

K Foord UMN Extension

Photo 1: Browning

K Zuzek UMN Extension

Photo 2: Bleaching on 'Green Mountain' boxwood

J Weisenhorn UMN Extension

Photo 3: Snowline winter burn

Evergreen trees and shrubs provide a lot of interest in our landscapes during Minnesota's long winters. Boxwood and rhododendrons are examples of the few broadleaved evergreens grown in Minnesota. Most of our evergreen species are narrow-leaved conifers - pine, spruce, arborvitae, juniper, hemlock, and yew - that have needles or scales for foliage. Unfortunately all of these species can be injured by winter burn and the winter of 2013-14 is proving to be no exception. Winter burn injury has been observed on many evergreen trees and shrubs. As we move from late winter into early spring and temperatures continue to increase, more of this damage will probably appear and existing damage will become even more dramatic.

Symptoms of winter burn are browning (Photo 1) or bleaching (Photo 2) of foliage, particularly on the south, southwest, and windward sides of plants. Evergreen foliage buried under snow is usually protected from damage while plant parts above the snowline are often damaged (Photo 3). In severe cases, the entire plant may turn brown and die. Sometimes symptoms occur immediately as damage occurs, but often symptoms appear or worsen as temperatures rise in late winter and early spring.

Winter burn results from water loss in plants during winter.

During the growing season, water is absorbed and pumped from soil into the roots of plants. From there, streams of water are suctioned up into a plant. Some of this water is used for plant growth and reproduction. But the vast majority is lost during transpiration as water is released back into the atmosphere through small openings called stomates on the lower surface of leaves. This lost water is quickly replaced as roots continue to absorb and pump water from soil into plants.

As plants acclimate and prepare for winter, deciduous trees and shrubs drop their leaves with their thousands or millions of stomates. But because evergreen woody plants retain their foliage, transpiration and loss of water from stomates continue during winter. This is especially true on sunny and windy winter days when higher temperatures and wind speeds increase transpiration rates. Because roots in frozen soil have no ability to replace this water, winter burn occurs as leaves desiccate, die, and turn brown. In more severe cases, buds, stem tissue, or entire plants may die.

Wait until spring before deciding how to care for your winter burned plants. If leaves are dead but buds and stem tissue near dead foliage are still alive, new plant foliage will regrow to replace winter burned foliage. On the other hand, if buds and stem tips were damaged, branches should be pruned back to ¼" above a bud in the live portion of the plant. In severe cases the entire plant may have died and plant removal and replacement will be necessary.

K Zuzek UMN Extension

Photo 4: Burlap protection

To prevent winter burn in the future:

  • Keep evergreens properly watered throughout the entire growing season until ground freezes. Initial soil freeze in Minnesota typically occurs in late November and early December .

  • Maintain a 3-4" layer of organic mulch around evergreens to help retain soil moisture throughout the growing season. As this mulch breaks down, it will also improve your soil's moisture holding capacity.

  • Protect plants in highly exposed sites during winter with burlap (Photo 4), lathing, snow fencing, or other materials to prevent too much exposure to sun or wind.

  • During future planting, properly place evergreens in the landscape by planting them on the eastern or northeast sides of a building. Avoid planting evergreens on the south or southwest sides of buildings or in any site with high exposure to winter sun and wind.

  • Plant evergreens in spring and water them consistently throughout the growing season so that they can use the entire growing season to establish well and expand their root system and water-absorbing capacity.

Snow fleas are conspicuous but harmless

Jeff Hahn, Extension Entomologist

Jeff Hahn, University of Minnesota

Photo 1: Snow fleas during late winter.

As temperatures warm and snow melts during later winter, a curious insect is sometimes observed. Snow fleas (Hypogastrura nivicola) are small (about 1/10th inch long). But because they are black and typically congregate in large numbers, they are very conspicuous against the white background of snow. From causal observation, snow fleas can look like black pepper. Additionally, they jump which helps to correctly identify them. Watch for snow fleas especially around the base of trees.

Their ability to jump leads some people to believe that these insects are true fleas. However despite its name, a snow flea is a type of springtail and does not bite people. Springtails are wingless and move by walking and jumping. They are very abundant although people usually don't notice them (except of course for snow fleas). They are often found in leaf litter and in the soil where they feed on decaying organic material, fungi, and pollen.

If you find springtails in your yard, just ignore them. They do not damage plants and are harmless to people. They are a curiosity that will go away on their own.

March 3rd Issue

Karl Foord - Extension Educator, Horticulture

I have a Meyer lemon (Citrus × meyeri) which goes outside in the summer and struggles through the long Minnesota winter. This is a wonderful plant with fragrant blossoms and nice fruit. I noticed a lot of very small white cottony blobs on the stems and leaves as well as some scale insects. I believe I have California red scale (Aonidiella aurantii). According to Whitney Cranshaw in his Garden Insects of North America this scale affects many ornamental plants and can be a serious pest of citrus. The adult females are round, reddish orange, and have concentric rings on the cover. Young stages produce some cottony filaments of wax around their body; later stages form the more solid cover.

This scale insect has a fascinating life cycle that features a sessile female and a winged male (Exhibit 1).

I clipped a citrus leaf and examined it under the dissecting scope. I found a sessile female (photo 1) and when I turned her over several crawlers exited (photos 2 & 3). I also found
white caps (photos 4 & 5), late first instars (photo 6) and an individual in first molt.

UCANR Publication #21529

Exhibit 1: California red scale (Aonidiella aurantii) life cycle from

Karl Foord

Photo 1: California red scale (Aonidiella aurantii) sessile female

Karl Foord

Photo 2: California red scale (Aonidiella aurantii) overturned female with exiting crawler

Karl Foord

Photo 3: California red scale (Aonidiella aurantii) overturned female with exiting crawler

Karl Foord

Photo 4: California red scale (Aonidiella aurantii) white cap first instar

Karl Foord

Photo 5: California red scale (Aonidiella aurantii) white cap first instar

Karl Foord

Photo 6: California red scale (Aonidiella aurantii) first instar

Karl Foord

Photo 7: California red scale (Aonidiella aurantii) first molt

Such a lifestyle made me curious as to how such species mate (Exhibit 2).

UCANR Publication #21529

Exhibit 2: California red scale (Aonidiella aurantii) mating

To eliminate the scale, I put the pot in the shower and try to hose off the scale. This knocks them down but does not eliminate them. This is probably an excellent time to consider the use of imidacloprid as a systemic insecticide. The plant is showing some flowers now and no pollinators will have access to these flowers. In addition the material will only be in the pot in a plant holder box in the house and so no chance of contamination of our water systems.

For more information see the excellent publication from the University of California Division of Agriculture and Natural Resources.

Life stages of California red scale

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