When is cold too cold for plants?

Very cold air is locked in place over Kansas and many gardeners wonder what might be in store for their landscape plants.  So will the cold affect our trees, shrubs and perennials?

It depends

There isn’t a simple answer and we won’t know exactly how our plant are impacted until spring when we can observe any damage. Cold injury to landscape plants actually depends on a host of variables such as plant hardiness, site and soil characteristics, weather fluctuations, past care, and the plant’s own health.

In Kansas, low temperatures alone usually aren’t our only problem.  Rather, it tends to be the extreme fluctuations of temperatures that can lead to cold injury.  Early fall or late spring hard freezes often cause cold injury, but even drastic mid-winter shifts in temperatures can be cause for concern when a stretch of mild weather is quickly replaced by the extreme cold. The current cold snap is a reminder of how quickly and drastically temperatures can change.

Cold injuries

Cold injury can take many forms depending on the type of plant and how the damage occurred.  In many cases cold injury may not even appear until spring or even later.  Symptoms such as cracked, split, or peeling bark, failure to leaf out, sudden plant death, missing blooms, or discoloration on evergreens may be seen if plants are damaged.

Sunscald damage (above) can often be prevented by using a tree wrap or trunk guard until young tree bark matures

Sunscald is a bark injury that is common in Kansas, especially on younger, thin barked trees.  This occurs when the sun warms up one side of the tree (south or west side) and causes it to ‘wake up’.  This warming action can cause a loss of cold hardiness in the bark tissue resulting in cells becoming active. These cells then become susceptible to lethal freezing when the temperature drops at night. The damaged bark tissue becomes sunken and discolored in late spring. Damaged bark will eventually crack and slough off.

Frost-heaving is another type of injury that occurs when repeated freezing and thawing of the soil lifts plant roots and exposes them.  Fall planted perennials can be easily damaged this way.

Desiccation (water loss) can also injure plants and kill plant tissue during extreme cold events.  Plants (especially evergreens) lose water to cold and dry winds faster than they can take it up from frozen soil.

What to do

Protecting landscape plants from cold injury begins before planting.  It is vital to choose plants for the landscape that are hardy to zone 6a or colder for Central Kansas. If temperatures fall below -10 degrees for a length of time with this cold snap there is a possibility even zone 6 plants could be damaged.  Plant labels will list the hardiness zone rating for the plant you are purchasing.  Plants native to Kansas are generally much less likely to be damaged but even they are not always immune to the affects of extremes.

Provide proper plant care all year long:  Make sure plants have the sunlight and moisture they prefer during the growing season and are fertilized and pruned at correct times to enter winter in the best possible condition.

Avoid excessive late summer/early fall pruning and fertilization:  These practices can inadvertently stimulate new growth on landscape plants which can delay the hardening off process they must go through to protect themselves from winter cold.

Protect plants:  Certain plants will benefit from winter protection.  Young trees should be protected with a trunk guard to shade the trunk and prevent winter and summer sunscald.  Mulching around plants is a great way to help minimize drastic soil fluctuations around plant roots.

Check soil moisture:  Ensure plants enter winter with good soil moisture and recheck and irrigate as needed on warm days during mid and late winter to prevent desiccation.

Snow is a good thing:  Snow cover acts like a natural insulator as well and can help moderate soil temperatures fluctuations.