Yard and Garden: How to Manage Garden Plants That Break Dormancy Early

Yard and Garden: How to Manage Garden Plants That Break Dormancy Early

Snow on daffodils.

AMES – Recent warm temperatures across Iowa have many gardeners concerned as they see the buds on trees and shrubs swell and break, and the leaves of spring perennials and bulbs appear in February, much sooner than they normally would. This puts plants at greater risk of damage if temperatures return below freezing in March or April. In this article, horticultural experts at Iowa State University Extension and Outreach offer advice on what to do in your garden when plants come out of dormancy earlier than expected.

What causes plants to break dormancy early?

Garden plants go dormant in the fall in response to day length and temperature. As temperatures drop and nights become longer, hormones change within the plant, causing plants to gradually adapt to cold winter temperatures and enter a state of dormancy. Plants will remain dormant until specified temperature requirements (measured using “chilling hours” and “degree days”) are met. This relatively complex and not yet fully understood system helps prevent plants from emerging from dormancy too early during the mid-winter warm-up period.

Each type of plant has its own cooling requirements that must be met before it breaks dormancy. A mild winter can allow plants to meet dormancy requirements earlier than usual. This makes them more likely to break bud sooner when abnormally warm temperatures occur in February or March, increasing their potential for cold damage.

What will happen to spring bulbs that appear too early in the season?

Tulips, daffodils, and other spring-blooming bulbs usually start emerging from the ground early in the growing season. Mild winter weather can encourage these plants to emerge early. This early appearance is often seen in areas warmer than the rest of the yard, such as the south and west sides of homes.

Although early foliage is undesirable, the risk is not as great as it may seem. Spring foliage can withstand cold temperatures. If temperatures return below freezing after foliage emerges, additional protection is usually not needed. Cold temperatures will retard growth and the leaves will usually have little or no damage. If the leaves are damaged, it is often nothing more than a few brown tips and leaf edges, which is unsightly but not a problem for the overall health of the plant. A snow blanket is especially useful in protecting leaves from extreme cold.

If flower buds appear early, they may be damaged or destroyed if temperatures are below 20 degrees Fahrenheit. A layer of organic mulch, such as straw or pine needles, can be placed around the base of emerging plants to help protect flower buds and slow growth by insulating the soil and regulating temperature fluctuations. Remove the mulch once normal temperatures return.

What can I do to help perennials that emerge too early?

When below freezing temperatures occur after a warm winter period, the extent of plant damage depends on the temperature, plant species, exposure, and plant growth stage. Many perennials will see little or no damage to their foliage, especially early-emerging, cold-tolerant perennials such as bleeding heart, daylily, mint, and columbine. If temperatures become too cold, damage may occur, causing leaf tips, edges, or entire leaves to turn white, brown, and/or collapse.

If temperatures are expected to drop into the mid-20s or teens, perennials can be protected with a layer of organic mulch, such as sawdust, straw or pine needles, that is several inches thick. Mulch will help insulate the soil, regulate temperature fluctuations, slow growth and prevent frost buildup. Watering will also be beneficial if the soil is dry and not frozen. Tulips appear at the end of February.

Most established perennials damaged by freezing temperatures should survive without any long-term damage. Plants with mild damage will continue to grow, and the damage will be hidden or can be minimized. The roots and crowns of severely damaged perennials will send out a second wave of growth, but the plants will be smaller than usual this summer. Good care in spring and summer (for example, watering weekly during dry weather) will help the plants recover.

How can I prevent damage to trees and shrubs that break dormancy too early?

There are no practical or effective efforts that home gardeners can take to prevent freeze damage to early emerging flowers or foliage on woody plants. It is impossible to prevent plants from “wake up” early. Flower buds that begin to swell or flowers that appear earlier than usual due to unseasonably warm temperatures may be distorted or destroyed when cold temperatures return. This is especially true of early flowering species such as magnolia, forsythia, rhododendron and azaleas.

Early-emerging foliage is also susceptible to freeze damage. Symptoms include wilting, browning or blackening of leaf tissue. Damaged growth often becomes weak and eventually falls from the tree or shrub. Damage appears to be most severe on species such as Japanese maple, hackberry, ginkgo, hydrangea, oak, and black locust.

Fortunately, temperatures below freezing will not have any long-term damage to the overall health of a healthy tree or shrub. Losing flowers for the season is unfortunate, but provided the weather does not become abnormal again, flowers can be enjoyed next year. Trees and shrubs can leaf again if the initial growth is damaged or destroyed, and new growth will appear later in the spring when normal temperatures return. Good care during the remainder of the year, such as watering during dry periods, should help recover woody plants grown during the last three to five years. Fertilizers are not recommended to aid recovery from hail damage.

How will abnormally warm temperatures in February and March affect my fruit trees?

Unseasonably warm temperatures early in the growing season can cause premature bud swelling, flowering or leaf emergence in fruit trees, especially apricots and peaches, which bloom earlier than apples, pears and tart cherries. As flower buds swell, they become more vulnerable to cold temperatures. They are most susceptible to infection before, during and after flowering.

If freezing temperatures return after a warm winter period, the amount of damage depends on the amount of early growth. The lower the temperature and the more buds and flowers develop, the more likely they are to be damaged or destroyed. If the flowers are damaged, this year’s fruit crop may be lower than usual. However, the trees themselves should not be seriously damaged. There are no practical efforts home gardeners can take to prevent freeze damage. (Commercial fruit growers may use wind turbines or water spraying to prevent frost damage, but these efforts are not practical for home gardeners.)

While losing most of the year’s fruit crop is disappointing, good care during the remainder of the year will help the trees recover and perhaps produce a bumper crop the following growing season.

Can I still prune?

In a year with more typical weather conditions, February and early March (late dormant season) are an ideal time to prune. Even with warm temperatures, plants can still be pruned until the buds burst. However, because trees and shrubs show signs of breaking dormancy, such as swollen buds or the appearance of foliage or blooms, pruning should be delayed until the new growth has fully expanded (mid to late May). Late spring/summer pruning is perfectly acceptable in all cases except oak trees, which should only be pruned during the dormant season.

In rare cases, late cold snaps can damage branch tips. Dead branches can be removed as soon as you notice them, but make sure they are dead (i.e. dry and brittle) before making any pruning decisions.

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