What does a warm, dry, windy winter mean for Wyoming landscape plants?

Many parts of Wyoming have had unusually warm and dry weather this fall and winter. If this trend continues and brings with it typical western region yo-yoing temperatures – warmer spells followed by deep freeze events – the state is likely to see a variety of effects on plants as spring and summer roll around. 

Here are some issues which might be encountered this spring and summer and what, if anything, to do about them.

Winter desiccation

Winter desiccation, also known as winter burn, tends to be a problem on evergreen trees and shrubs across the state. 

These plants keep their leaves or needles all year and continue to lose some water during the fall and winter. Dry, windy weather conditions in winter increase this water loss to the point where tissue damage can occur and leaves and needles turn partially or wholly brown – usually from the tip of the needle downwards.

This type of damage is often unevenly distributed around the tree or shrub, with the sides facing the sun or prevailing winds exhibiting more damage. 

Damage can be reduced by watering unfrozen bare ground during extended warm spells or using temporary windbreaks to protect small trees. When planting new trees, damage can be reduced by choosing appropriate species for the site.

Sunscald

Sunscald is a condition where tissue on tree trunks becomes damaged. It often appears as an area of shrunken, cracked dead bark on a tree trunk. Sunscald appears to be caused by warm sun shining on the bark, which causes cells to become more active. 

As the temperature drops again at night or during a cold spell, these active cells can freeze, causing tissue damage and death. The damage will be found on the side of the tree facing the sun – usually south to southwest. 

Trees will try to partition off this type of damage and may continue to grow reasonably well or they may die from severe damage. 

Sunscald can be prevented by not pruning off too many lower branches, wrapping thin-barked younger tree trunks with white tree wrap in fall or using types of white paint on tree trunks to reflect the sunlight.

General cold damage

During fall and winter, perennial plants are in a condition called dormancy. The factors controlling plant dormancy can be complicated and vary amongst different kinds of plants.

For many species, cold temperatures are not the only factors driving plant dormancy.

In many deciduous trees, dormancy is driven by both day length and temperature, among other variables.

As plants emerge from dormancy and start putting on new growth, they may experience cold damage. Cold tolerance varies depending on the kind of plant – species or even variety – and its stage of growth.

Different parts of plants also have varying degrees of resistance to cold damage. Cold damage can express itself in several ways.

In general, damage to flowers becomes more likely if they are closer to blooming and temperatures are colder. 

Flowers tend to be most susceptible to cold just before, during and just after bloom. However, a flower’s cold tolerance is greatly dependent on plant type. 

Many daffodil flowers can take significantly more cold without injury than apricot blossoms, for example. 

If damaged, flowers may become limp, change color, turn brown and fall off.

If leaves are injured due to severe cold, they will eventually turn brown or black and possibly fall off. If it is early enough in the season, the plant will expend extra energy reserves to replace them. The same goes for many herbaceous stems.

If severe cold after warm spells damages a branch, it may die back partially or completely or wounds may be created in the branch’s tissue. If a branch dies back partially, dormant buds in the surviving tissue may become active and start to grow. 

Wounds make the plant more susceptible to infection by other organisms. 

If the plant is in reasonable shape, it will try to repair or partition off these wounds before an infection occurs.

Perennial flowers and foliage plants

Many healthy perennials which sustain cold damage early in the season will outgrow it as the season progresses, replacing damaged leaves and stems.

People often become concerned when they see spring bulbs emerging from the soil earlier than normal. 

In general, many spring-blooming bulbs are adapted to quite cold temperatures. However, cold tolerance varies among species.

If cold damage occurs, the extent will vary by plant type, stage of development, amount of snow cover – it can act as an insulator – how low the temperature drops and length of the cold spell.

If bulb flowers and foliage are damaged by cold, do not cut them off. Leave them in place until one can see the full extent of the damage – usually by early summer – or they turn fully brown and crispy. 

Any green foliage left surviving will photosynthesize, providing the bulb with the energy it needs to survive and hopefully recover, grow and bloom again next year.

Newly emerged foliage of other perennial plants is often more cold hardy than older foliage. In addition, crowns and roots are often insulated by the soil. 

If perennial leaves do sustain damage, they will change color and eventually brown. When new foliage grows out, it can sometimes hide this damage.

Trees and shrubs

As mentioned previously, cold damage depends on the species and variety of the tree or shrub, its stage of plant growth, the low temperatures reached and how long those temperatures persist. Most trees and shrubs in Wyoming weather temperature swings reasonably well.

In some circumstances, emerged deciduous tree leaves will sustain enough damage they will drop, and the tree will expend reserves to replace them. If trees are stressed from drought – especially multiple years of drought – or other reasons, they have fewer reserves to call upon for this purpose.

The flowers of certain early-blooming trees and shrubs may sustain freeze damage which will cause them to turn brown and become limp and mushy.

Cold damage is fairly common in fruit trees in Wyoming and will cause the fruit blossoms to abort and fall off, resulting in little or no fruit set for that growing season. This phenomenon has been studied in commercial fruit crops, and various charts exist showing the likely effects of different temperatures during particular stages of flower development. 

Losing this year’s blossoms will not harm the tree’s ability to set fruit in following years. It just affects the yield for this year.

Branches or stems can also suffer cold injury. If this kind of damage has occurred, wait and see how the plant responds – mid- to late-summer at the earliest, waiting a year is often better – before getting out the pruners. 

Trees and shrubs often have reserve buds which will start to activate if a primary bud has been damaged. 

Trees can be slow to show certain types of damage, so patience and observation may be needed before all damage is apparent. Unless there are safety concerns, it’s best to monitor trees and shrubs as spring and summer progress to see what the true damage is.

Some trees and shrubs are not well adapted to many parts of Wyoming. Trees such as peach trees may survive for a number of years before a bad winter or untimely cold spell severely damages or kills them. 

Keep this in mind when selecting trees and shrubs for planting or replanting. 

Choose trees rated at or below an area’s U.S. Department of Agriculture (USDA) Hardiness Zone. Below is preferable. Those who live in areas rated as USDA Zone Five should select trees with at least a Zone Five rating, but Zones Three and Four are preferable.

Assisting plants

In general, plants can recover from quite a bit of damage when they have the resources they need. 

Once temperatures are above freezing, make sure to provide enough water and nutrients to sustain the plants during their recovery period.

During warm dry spells in fall and winter, consider watering trees and shrubs – giving evergreens and newly planted trees priority – when temps are above 40 degrees Fahrenheit and the soil is dry. If it has been very dry, warm and/or windy, consider watering other plants as well.

Plants do not need a lot of water this time of year. Watering once a month is often sufficient. Make sure to water early enough in the day to allow the water to soak into the soil before freezing temperatures return. Also remember to drain hoses and spigots each day after watering.

Once spring has sprung and plants are growing quickly, water all plants consistently. Mulch trees and shrubs and make sure they are not suffering from nutrient deficiencies – often appearing as oddly colored leaves – or other issues hindering their recovery.

With annuals, remember to practice patience and vigilance. Warm spells in early spring can tempt individuals to plant annuals such as flowers or vegetables a bit too early. 

Whether one chooses to do this or not depends on their risk tolerance and how much effort they want to invest in plant protection and/or replacement. Sometimes early planting pays off, but often it doesn’t.

Jennifer Thompson is a University of Wyoming (UW) Extension horticulture specialist. This article was originally published by UW Ag News.