Spring freezes are a common occurrence in these parts, but Denver’s low temperature of six degrees on April 10 was a record breaker. The sudden temperature drop and prolonged cold caused plenty of plant injury including many species normally unfazed by spring cold snaps. Most plants will recover, however the injury is much more severe than we normally experience. Plants including fruit trees, forsythia, lilacs, euonymus and newly emerged flowering bulbs soon withered. Broadleaf evergreen plants are the most susceptible to injury. These plants are not widely planted in Colorado but include boxwood, bamboo, and euonymus. Next in line for injury are leafy plants (deciduous) that lose their leaves during the winter and lastly needle bearing plants including arborvitae, juniper, pines, and spruce.
In the Plant
During spring freezes, ice crystals form within plant cells disrupting membranes. Plant cells loose water and localized tissues suddenly dehydrate. The severity of the damage differs with plant species, genetics, age of the plant and hydration prior to the cold weather.
What Does the Damage Look Like?
Deciduous:
The foliage became flaccid, water-soaked and finally black and wilted. Twigs became water-soaked and dry.
Evergreens:
Foliage rapidly turned tan, red and brown. Twigs dry and become brittle.
Here is What You Can Do Now.
Evergreens:
Injured plants are mainly small trees and shrubs.
Arborvitae, Manhattan euonymus, boxwood,
and select juniper, pines and spruce
Using a rake remove most of the leaf/needle litter from underneath the plant. Only a couple of inches of mulch or duff should remain to cover the soil. If the injured plant is ten foot tall or less use a straight stream nozzle on your garden hose and spray the foliage removing some of the dead and loose needles. Loosen the foliage to get additional light into the interior. Hand water next to the plant trunk 3 – 5 gallons of water a week. Watering small evergreen plants is especially beneficial as this area is often dry even during wet periods. This will encourage new growth. Reassess the plant in the first part of June. Severely damaged plants may need to be removed while others may just need to have some dead foliage and twigs pruned.
Deciduous:
Most of these trees and shrubs will recover. Some trees such as oak that normally leaf out late in the spring will be just fine. Ash trees which already attempted to leaf out and got froze have not yet begun to put out new leaves. Shrubs such as elderberry, forsythia and lilac are producing new leaves via sprout (sucker) growth. This growth along with the dead twigs will need pruning. Again, evaluate the pruning needs during the first part of June.
It may be several weeks to a month before we know the full extent of the damage and what new growth will sprout from seemingly lifeless branches. Mulching around
the base of plants and fertilizing to encourage root growth is beneficial.
Supplemental watering of trees and shrubs outside of the lawn irrigation is also encouraged. Insect and disease control treatments should continue as normal.
Plants injured this spring may show additional injury later in the growing season, should the summer prove to be hot and dry. This damage may manifest in off color foliage, premature defoliation and additional dead twigs and branches.
If you have any questions or concerns, please contact Swingle tree and lawn care experts can provide guidance.
