SPRING FREEZE UPDATE FROM MISSOURI BOTANICAL GARDENS
Three weeks after the great Spring Freeze of 2007, we’re seeing encouraging signs of recovery throughout our gardens, reaffirming our faith in the remarkable resiliency of plants and nature.
The question on everyone’s mind is “What can I do to help my trees and shrubs recover?” Like a broken record, the answer remains that the best option for now is to continue to exercise patience and restraint where pruning and fertilizing are concerned until the full extent of dieback has been determined. Provide water if rainfall is lacking. The growing season is in full swing and most plants respond best to one good soaking per week, with the classic 1-1½ inch as a standard benchmark. To maintain even moisture levels and regulate soil temperatures, cover root zones with a shallow 2 to 3-inch layer of mulch if this has not yet been done.
Only in recent days has new growth begun to push out from Japanese maples, magnolias, oaks, ginkgos, and other trees suffering extensive damage. Fast-growing trees such as hard maples are recovering more quickly than slower-growing species such as oaks. In many cases, well-established younger trees are recovering faster than older specimens of the same species. This is normal, given the circumstances. Older trees that were healthy prior to the freeze still have an excellent prognosis and should show marked improvement by the end of May.
Some Saucer and related hybrid magnolias suffered damage to stem tips that has caused variable dieback that will require corrective pruning once we fully ascertain what’s alive and what’s dead in the weeks ahead. Loebneri hybrids, including the popular ‘Leonard Messel’ and ‘Merrill’ fared surprisingly well with only minimal damage.
While new growth is a welcome sight, I am concerned about some Japanese maples. The first growth of many varieties froze and has dried up; if not pruned off, it will eventually fall off. There may also be hidden damage to the vascular tissue beneath the bark that could result in additional dieback when summer stresses arrive in full force. I expect pruning to remove dead wood and restore aesthetic balance will be necessary later this summer and possibly for years afterward. This freeze event was one that will weed out marginally hardy plants, and there could be some casualties.
Many callers are also concerned about azaleas, crape myrtles, and Bigleaf hydrangeas. Damage to azaleas has been highly variable, with some varieties showing minimal injury, near-normal flowering, and vigorous new growth. Flowers and buds that froze on the more tender varieties should be left to fall off naturally rather than pulling or pruning it off and risking inadvertent damage to live tissue present at the base of frosted growth.
Crape myrtles were at such a vulnerable stage of growth that many were killed to the ground when their sap froze and expanded, damaging vital cambium tissue. This will be a shock to gardeners who have become used to large specimens, courtesy of the string of unusually mild winters in recent years. All dead wood should be removed as soon as new growth appears. Though crape myrtles bloom on new growth and we look forward to their showy displays, the most severely damaged varieties may not sprout before June and could show poor flowering this summer.
Bigleaf hydrangeas (Hydrangea macrophylla) are marginal shrubs whose popularity has also benefited from recent mild winters. I have yet to see a specimen this spring that has not suffered extensive damage to their old growth and anticipate few, if any flowers on most cultivars this summer. New growth has already occurred from the base of most varieties, so dead stems can be pruned back any time now. This summer should be a good test for the new generation of remontant (repeat-blooming) cultivars said to be capable of producing flowers on new wood. We’ll find out if they stand up to all of their advance media billing and prove productive under this most challenging of weather circumstances.
Roses present unique challenges also, especially to inexperienced growers. Most of the popular new landscape shrubs will recover nicely, provided they were well-established before the freeze. Many roses that bloom on old growth, including most climbers, have suffered loss of flowering wood and will have few, if any, blooms this year. Considerable pruning of dead growth and training of new shoots this summer will be necessary to re-establish a sturdy framework of stems to support future growth. For roses that flower on new growth, especially hybrid teas, the issue of survival may come down to whether gardeners left mulch in place or succumbed to the temptation to remove it prematurely before the traditional average last frost date of April 15. With blackened stems all the way to the mulch and soil lines, there may still be live dormant buds below grade mustering the energy to answer the call to active growth. The only way to find out is to wait and see and only experienced growers may have the patience to do so. When pruning is done, look for the tell-tale ring of live green cambium tissue and absence of darkened and discolored interior wood when examining cut stems. Otherwise, it may be necessary to cut further down on stems until unblemished tissue is revealed. Begin to fertilize roses once pruning is completed.
It’s apparent that some herbaceous perennials fared better than others. For plants with only minimal injury it’s best to leave the partially green leaves for now and remove only dead growth. Perennials will benefit from a light fertilization at this time to speed their recovery.
Many gardeners have expressed concern about iris, peonies and daylilies. It’s apparent that the Garden benefited from the “heat island” effect, as our iris and peony collections fared remarkably well and we look forward to showy displays to greet our Mother’s Day visitors. Likewise, we’re expecting normal displays from our daylilies later in summer—weather provided, of course!
It’s important not to let down your guard, so scout your garden regularly for new signs of trouble and take corrective action promptly. Insect and disease pests may take advantage of the weakened condition of recovering plants to gain the upper hand. Injured plants can ill afford additional setbacks. Proper identification of symptoms is the first step in remedying problems. Take advantage of our walk-in Plant Doctor diagnostic services, staffed by trained Master Gardeners. This service is available daily except Sunday between 10 am and 3 pm in our Kemper Center for Home Gardening. If treatment is necessary, consider hiring a certified consulting arborist to provide professional services. Seek out qualified arborists certified by the International Society of Arboriculture. Some can be found in the Yellow Pages under Tree Service or they can be located through the society web site http://www.isa-arbor.com/findArborist/findarborist.aspx.Be sure to select an arborist employed by a private tree care firm rather than a utility or municipality.
Chip Tynan Missouri Botanical Garden “Answer Man” William T. Kemper Center for Home Gardening
P.S. For specific inquiries, call the Garden’s Horticultural Answer Service, Monday through Friday, 9 a.m. to noon, at (314) 577-5143, or visit the Gardening Help section of the website at www.mobot.org.
P.S. Patience, Patience, Patience. This is the most important aspect of the entire situation. Most of the plants will come back (maybe not as lush) but most will eventually resprout. Please, don't remove a perfectly live tree or shrub now. Chances are, it will soon revive.