Snow on the daffodils.
AMES – Recent warm temperatures across Iowa have many gardeners concerned as they see the buds on trees and shrubs snap and break, and the foliage of perennials and spring bulbs emerge in February, much earlier than the habit This puts the plants at a higher risk of damage if sub-freezing temperatures return in March or April. In this article, horticulturists with the Extension and Extension of the State University of Iowa advise what to do in your garden when plants break dormancy earlier than expected.
What makes plants break dormancy earlier?
Garden plants enter dormancy in the fall in response to day length and temperature. As the temperatures cool and the nights get longer, the hormones in the plant change, causing the plants to gradually acclimate to the cold winter temperatures and go dormant. Plants remain dormant until specific temperature requirements (measured with “chilling hours” and “degree days”) are met. This relatively complicated and not yet fully understood system helps prevent plants from coming out of dormancy too early during a midwinter warm-up.
Each plant species has its own cooling requirement that must be met before breaking dormancy. A mild winter can allow the plants to meet their dormancy requirements before use. This makes them more likely to break buds earlier when abnormally warm temperatures occur in February or March, leading to a higher probability that they will experience cold damage.
What happens to spring bulbs that emerge too early in the season?
Tulips, daffodils and other spring flowering bulbs typically begin to emerge from the ground early in the growing season. The mild winter climate can also favor the early emergence of these plants. This first emergence is more often seen in areas that are hotter than the rest of the yard, such as in the south and west side of the houses.
While the premature emergence of foliage is undesirable, the danger is not as great as it seems. The foliage of spring bulbs can tolerate cold temperatures. If below-freezing temperatures return after the foliage emerges, no extra protection is needed. Cold temperatures retard growth and leaves usually have little or no damage. If the leaves are damaged, it is often not more than a few brown tips and leaves, which is unsightly, but not a problem for the general health of the plant. A blanket of snow is particularly useful to protect the leaves from extreme cold.
If the flower buds emerge early, they may be damaged or destroyed if the temperatures are colder than 20 Fahrenheit. A layer of organic mulch, such as straw or pine needles, can be applied around the base of emerging plants to help protect the flower buds and slow growth by insulating the soil and regulating fluctuations of the temperature. Remove the mulch once the temperature returns to normal.
What can I do to help perennials that emerge too early?
When temperatures below freezing occur after a warm winter period, the extent of plant damage depends on temperature, plant species, exposure and plant growth stage. Many perennials will show little or no damage to the foliage, especially the first-rate, cold-tolerant perennials such as bleeding hearts, daylily, catmint and columbine. If the temperature is too cold, damage can occur, causing the tips of the leaves, the edges or the whole 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 wood mulch, straw or pine needles, spread several inches thick. . Mulch will help insulate the soil, regulate temperature fluctuations, slow growth and prevent frost. Water if the soil is dry and unfrozen will also be beneficial. Tulips emerging in late February.
Most well-established perennials damaged by freezing temperatures should survive without long-term damage. Plants with light damage will continue to grow, and the damage will be masked or can be cut. The roots and crowns of severely damaged perennials will send a second flush of growth, but the plants will be smaller than used in the summer. A good care this spring and summer (for example, watering every week during dry weather) should help the plants to 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 the first emerging flowers or leaves on woody plants. It is impossible to prevent the plants from “waking up” too quickly. Flower buds that begin to swell or flowers that emerge earlier than normal due to unusually warm temperatures may be disfigured or destroyed when cold temperatures return. This is especially true for early flowering species such as magnolia, forsythia, rhododendrons and azaleas.
Early emerging foliage is also susceptible to freeze damage. Symptoms include rusting and browning or blackening of leaf tissue. Damaged growth often becomes limp and eventually drops from the tree or shrub. Damage appears to be most severe on species such as Japanese maple, hackberry, ginkgo, hydrangea, oaks and black locust.
Fortunately, below-freezing temperatures will not do long-term damage to the overall health of a healthy tree or shrub. The loss of flowers for the season is unfortunate, but as long as the weather is not abnormal again, the flowers can be enjoyed the following year. Trees and shrubs can leave again if the initial growth is damaged or destroyed, and new growth will emerge later in the spring when normal temperatures return. Good care during the rest of the year, such as water during dry periods, should help the recovery of wood plants planted in the last three to five years. Fertilizer is not recommended to help recovery from cold damage.
How will the abnormally warm temperatures in February and March affect my fruit trees?
Unseasonably warm temperatures at the beginning of the growing season can cause premature swelling of buds, flowering or emergence of leaves in fruit trees, especially apricots and peaches, which bloom before apples, pears and cherries. As the flower buds swell, they become increasingly vulnerable to cold temperatures. They are most susceptible just before, during and after flowering.
If freezing temperatures return after a warm winter period, the amount of damage depends on the amount of premature growth. The colder the temperature and the more developed the buds and flowers are, the more likely to be damaged or destroyed. If the flowers are damaged, this year’s fruit yield may be smaller than usual. In any case, the trees themselves should not be seriously damaged. There are no practical efforts that home gardeners can take to prevent freeze damage. (Commercial fruit growers may use wind turbines or water spray to prevent frost damage, but these efforts are impractical for home gardeners).
While the loss of most of the fruit set for the year is disappointing, good care during the rest of the year will help the trees to recover and potentially produce an abundant harvest the following growing season.
Can I have plums too?
In a year with the most typical weather conditions, February and the beginning of March (late sleeping season) is an ideal time for pruning. Even with the hot temperatures, the plants can still be pruned until the bud. However, as trees and shrubs show signs of breaking dormancy, such as swollen buds, emerging foliage or flowering, pruning should be delayed until the new growth has fully developed (a mid to late May). The pruning of the end of spring / summer is perfectly acceptable in all cases, except for the eyes, which should be pruned only during the dormant season.
In rare cases, late frosts can damage the tips of the branches. Dead branches can be removed once they are noticed, but make sure they are dead (ie, dry and brittle) before making any pruning decisions.
