Ice Covered Stem

Those of us who live in the temperate zones i.e. those that routinely get at least one killing frost, often have to deal with injury to our landscape plants from cold temperatures. When we venture out into the garden in the early spring, we begin to see dead or damaged plants that seemed to be in good shape the previous fall. What happened?

Of course, the first thing and for some people, the only factor we think about is cold temperatures. However, there are several other weather related impacts that need to be considered.

1. Low Temperature Injury – Most gardeners are familiar with the USDA Hardiness Zone map. This divides each area of the country into units based on the typical lowest temperatures expected. For instance, if you are in USDA Zone 5, you can expect to experience winter low temperatures between -10 and -20 degrees Fahrenheit.

Plants that are not winter hardy for your Zone will suffer damage to buds, twigs, branches or, in extreme cases, the entire plant will die. Basically, this is due to the fact that the cells within the plant tissue will freeze and cause its solid cell walls to break. The dead cells will turn brown and, of course, will never return.

Desiccation on Rhododendron

2. Desiccation Injury – Cold air can hold less moisture than warm air. So, cold winds flowing over buds and other plant tissue in the winter may slowly pull moisture from the plant. Since the soil is frozen and the roots cannot replenish the moisture, the plant cells may lose all their water. This will cause death and browning of that tissue. This is especially a problem on broadleaf evergreens such as rhododendrons and boxwood.

3. Frost Cracks – When the wood on trees or shrubs is frozen but the sun shines on a January or February day, there may be enough heat generated to begin to expand the exposed bark. Since the wood beneath is unable to move, the bark may develop a split. Often, but not always, this split will be on the south or southwest side of the tree since that is the direction of the most intense winter sun.

4. Snow or Ice Damage – The weight of ice or snow may cause flexible trees such as birches or evergreens such a pines bend over. In some cases, this will be heavy enough to actually break off the branch or the entire tree. In others, however, it merely bends the plant and it will return to its formal position. Unfortunately, the bending may have cracked the bark allowing fungal organisms to enter the plant and cause the development of open wounds called cankers.

5. Salt Damage – Although special anti-icing products are now available that do not damage plants, plenty of people still use good old salt. Whether the salt is regular table salt or a granular fertilizer, it has the same effect on plants. When salt comes into contact with plant foliage or roots, it forms a powerful attraction to water. It will pull the water out of the plant tissue causing it to die and turn brown.

6. Heaving – Soils that contain a lot of clay are plagued by a process called heaving. Clay soils are poorly drained and retain too much water. During the winter, when that water freezes, it expands. This can cause damage to roots and, in some cases, may actually push plants out of the ground. The effect is greater on plants that were installed late in the fall and have not had a chance to establish enough roots.

7. Rodent Damage – Unlike bears, mice do no hibernate in the winter. Therefore, they need to continue to eat. If the winter has a lot of snow and other food sources are denied, mice may turn to chewing on the bark of trees. Often, they will burrow under the snow and much and eat the bark at the base of the tree. If they destroy enough of it, they will have girdled the tree resulting in its loss.

8. Early Spring or Early Fall Frosts – Plants that are hardy for cold winter areas have developed one or more techniques for surviving. Generally they build up hardiness slowly in the fall, reach peak hardiness in the height of winter and then slowly lose hardiness into the spring. For example, a plant buds that will survive in -20 degrees in February, may be damaged or killed by 20 degrees in October or March.

So, what can you do about all these problems? Check back next week for How to Avoid Winter Injury in Landscape Plants.

1. Low Temperature Injury – The key to avoiding this problem is to not “stretch the zone” with your plants. This means that, for instance, if you are in USDA Hardiness Zone 6, avoid using plants that are only hardy to Zone 7 or 8. You may get away with this for several years if the winters are warmer than average. However, all that it takes is one night below the “typical” low temperature for your zone and the plant may die. That is a fallacy of people when they think that global warming automatically changes their climate zone.  You may, in fact, experience above average temperatures for 364 nights but, if that one night in the dead of winter drops below the hardiness level of the plant…it will still be dead.

The other factor here is the condition of the plant coming off the previous growing season. There is some evidence that plants that were stressed by drought or excessive heat or other factors may not be able to attain their normal level of winter hardiness. Thus, even though the plant is technically hardy for a particular zone, if it comes into the winter under stress, it may still be damaged by “normal” low temperatures.

Burlap Wind Protection

2. Desiccation Injury – Generally, the key factor in desiccation injury is the type of plant and the direction of the predominate winter winds. As we mentioned before, evergreens, especially broad leaf evergreens like rhododendrons and boxwood, are most likely to sustain this type of damage. Therefore, these types of plants will benefit most from protection from winter winds. This can be done by a temporary wind shield such as burlap fences for rhododendrons and fences, buildings or other more permanent windbreaks may also be used.

Plastic Tree Wrap for Both Rodents and Frost Crack

3. Frost Cracks – Preventing the penetration of winter sunlight is the object to preventing winter cracks. Smooth, dark, young bark is most susceptible to this damage. So, on newly planted, small trees such as a maple, it might pay to wrap the bark in a light colored material that will reflect the rays of the sun. The key is to remove this wrap after a season or two and either leave it off or replace it if the bark has not yet become rough textured and more “mature”. If the wrap remains on the tree too long, the expansion of the girth of the tree may eventually cause it to cut into the bark and girdle the tree.

4. Snow or Ice Damage – First and foremost, don’t shovel the snow from your driveway or sidewalks onto the top of your landscape plants. Generally, they can handle the weight of a normal snowfall but the added snow may cause damage. If you have tall, upright arborvitae trees, you might need to tie them together with ropes in the fall  when they are older and have become very tall and weak. Gently knock off excessive snow cover as soon as the storm is over.

Wrap Left on Too Long

5. Salt Damage – If you must control the ice on sidewalks or driveways with other than mechanical means, always use products labeled as not damaging to plants. Remember that granular fertilizers are also salts and could damage nearby plants if applied excessively for ice control throughout the winter.

6. Heaving – This problem takes a little more work than some of the others. If you have clay soil, you need to incorporate large amounts of organic matter such as compost, leaf mould, clippings, etc. This will help to improve the drainage of such soils. However, this needs to be done before you install your plants. It is pretty tough…though not impossible…to amend the soil after landscape plants, other than annuals, have been put into the ground.

7. Rodent Damage – This is most commonly a problem during extremely cold winters with large amounts of snow on the ground for long periods. Since you can’t predict which winter is going to meet these criteria, you need to take some general steps. One is to avoid piling mulch up against the trunk of the tree. Often mice will burrow in the mulch right up to the tree and chew away in the comfort and security of the mulch. Also, wrapping the trunks of young trees that still have immature bark can also be a deterrent.

8. Early Spring or Early Fall Frosts – Unfortunately, on a practical level, there is not always anything you can do about this type of damage. If you have plants small enough to cover with a tarp or other covering, that can help for a night or two. However, this won’t do much for large shrubs or trees. Remember that the goal of any covering is to keep the temperature of the plant material at least one degree above that needed for the cells to freeze. Unlike animals, plants don’t “feel” the cold. Their cells either freeze and die or they do not freeze and survive. It is that simple.