By Laura Pottorff, Colorado State University Cooperative Extension agent, horticulture and plant pathology
"Why is my cottonwood tree losing its leaves in June?"
"My lilac's leaves have dark irregular dead areas on them. Is this a disease?
"I planted a honeylocust last fall. It leafed out nicely in April, but the leaves were killed by the late April freeze. No new leaves have emerged. How long should I give this tree?
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If you have similar questions about trees and shrubs, chalk them up to a late spring freeze.
When the weather is beautiful and warm in April, and homeowners begin to think about getting the porch furniture out, many trees and shrubs also sense summer in the air and begin to push out leaves.
How cruel of Mother Nature! Winter often returns in April and stays for a while. Temperatures can drop into the low 20's during the nights. While Rocky Mountain gardeners can go indoors, turn on the heat and pull up the covers, their plants are stuck with the cruel switch nature can play.
Leaves on many trees and shrubs can be damaged by the low temperatures. Among those trees are colorado spruce, green ash, honeylocust, and hackberry. These three trees have leaves just beginning to emerge fro the bud, which is the prime stage for freeze damage. Other trees and shrubs that leaf out earlier, such as crab apples, lilacs, linden, silver maple and Siberian elm, are not damaged, as their leaves are "older" and more hardy.
Was your tree damaged by a April freeze? If the young leaves are blackened, shriveled and dry as a bone, the answer is, yes. The answer also is yes if, during a present hot spell, leaves are dropping prematurely.
What will happen to freeze-affected trees? The answers vary. Freezing of new leaves will damage the trees slightly. It will set them back and possibly weaken them. Other effects depend on the health of the tree prior to a dramatic temperature change. Healthy trees are able to store three years worth of energy. If the tree was vigorous and healthy last year, it will put out a new set of leaves from what are called adventitious buds. This "secondary" growth often is weaker than the primary growth put out first, so expect to find small branchlet loss in high wind storms during the summer. The secondary growth also may come out in a bushy or tuft-like pattern called "witches brooming." The shape and form of these trees may look a little different for a couple of years. If the particular tree in question was declining prior to this temperature drop, expect to see further decline. Trees in this situation have less stored energy and may not recuperate as well as their counterparts. If a tree damaged by freeze has not re-leafed by June 1, it probably never will.
Homeowners can't do much to undo Mother Nature's blow. Prune out dead branches during summer and do not "baby" the trees with too much water. Overwatering in a heavy clay soil (which most of us have in abundance) will lead to oxygen starvation in the root system. The best rule of thumb for tree watering is: "deeply, less often". Your lawn will benefit from this watering rule of thumb, too. If the tree is watered with the lawn, try to accustom the lawn to receiving water less than every three days. Resist fertilizing until midsummer, but do it before July 30. Fertilization is not a must. Most trees in lawns receive enough.
Your best bet is to let those freeze-damaged trees recover on their own.
Photos: Judy Sedbrook
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010