By Adrian Higgins
Thursday, March 27, 2008
Those lovely cues of the dawning spring -- the smell of mulch, the sight of pale blossoms, the sweet softness of the greening grass -- all point to the celebration of a new growing season.
Enjoy the moment, for the year ahead will be a tough one in the garden.
We may have forgotten what happened last year, but our plants have not, especially the biggest of them. "It's hard to talk to someone about drought when it's pouring with rain," said entomologist Rex Bastian. "But the trees remember."
They remember weeks of little or no rainfall, of the soil hardening around their dying roots and the leaves turning and dropping early. The ill effects of one of the worst dry spells on record will be seen this spring and summer and may linger for years. Look for symptoms of delayed drought stress in your woody plants so you can help them, and be prepared for some to die.
"Trees don't react quickly and often tend to show effects after the damage has occurred," said Bastian, vice president of field education for the arborists group the Care of Trees.
Amid the splendid ornament and fertility of spring, spare an eye for old, arboreal friends, and be prepared for such symptoms as dead branches, stunted growth and worse. "A lot of symptoms probably won't show up until the summer," said horticulturist David Yost, who runs a plant clinic at Merrifield Garden Center in Fair Oaks.
Some are already evident, however. Yost said a customer arrived recently with a sad sample of an azalea bush bearing the effects of a September with just six-tenths of an inch of rainfall. "Growth was poor, color was poor and the flower buds had not set," he said.
Barbara Bullock, curator of the azalea collection at the National Arboretum, said she is not seeing so much bud loss in Washington's favorite spring flowering shrub as the death of branches. "Major branch dieback in mature azaleas," she said. "We have been pruning lots of dead wood."
Bastian said you don't have to remove every dead twig from a tree, but major limbs that are dying or dead should be removed, not just for the health of the tree but to prevent a hazard to people and property. The removal of large tree limbs is a perilous job best left to the professionals.
Canker diseases are another drought-related malady, seen as sunken areas beneath bark or as parts of the trunk eaten away. If allowed to spread, the decay can kill branches or a whole tree.
In addition, drought stress can bring on a root rot disease that, once present, will slowly spread and kill a tree. The fungus, armillaria, is also called shoestring fungus for the strands that kill roots and cause cankers at the base of the trunk.
One of the most damaging insect pests to drought-stressed trees is the borer, numerous species of beetles whose larvae tunnel into the vascular system of a tree and, if present in sufficient numbers, can kill it. A species called the two-lined chestnut borer is particularly pesky and targets oaks as well as chestnuts. Often, the damage is in the tree canopy and not apparent, but the insect can also attack the base of the tree, where it is even more lethal.
Michael Raupp, an entomologist at the University of Maryland, said the drought's timing couldn't have been worse for hardwoods attacked last spring by the gypsy moth caterpillar. Dry conditions in May and June were not conducive to trees refoliating. This double whammy of leaf loss and drought stress set up the ideal conditions for borers. In addition, gypsy moth egg masses surveyed over the winter portend an even worse outbreak this spring in much of Maryland.
"There are so many doomed trees right now," Raupp said. "They will push out leaves in the next month or so, but they are so heavily infested [with borers] they'll die over the course of the summer."
Systemic insecticides can be used to kill the larvae, but trees with excessive dieback in the canopy may have a compromised vascular system that cannot transport the pesticide to where it will work, Raupp said.
Of course, not all the damage will be as dramatic or irreversible, especially if the gardener follows some sound practices.
First, make sure that the root zone is covered with a two-inch layer of organic mulch, which will conserve soil moisture. Over several months, the mulch will decay and help feed the beneficial microbial life in the soil. Make sure that you, or your landscaper, avoid the common but harmful practice of piling mulch against the base of a tree trunk. The larger the mulched areas, the more effective the covering, because the feeder roots of a mature tree reach well beyond the canopy. A broad area of mulch also keeps turf grass away from trees. Fertilizers and pesticides used on lawns, as well as lawn watering regimes, may harm trees.
Also, do not apply excessive amounts of fertilizers to trees and shrubs this spring. Stressed trees given high nitrogen feeds can actually expend energy processing the nutrients, resulting in "a net decrease in the reserves of that tree," Bastian said.
And although you want to keep root zones occasionally and deeply watered when it turns hot and dry, over-watering could well rot the roots. Most trees benefit from an occasional deep watering in hot, dry weather; let the top inch or so dry out before returning.
The drought does bring a few silver linings: Populations of lawn grubs and black vine weevils should be down this year, Raupp said. And the crisis reminds us to make better decisions about what, and where, to plant this spring. The trees and shrubs most harmed by the drought were already compromised by growing in poor soil and in locations ill suited to the particular plant. Pick a plant suited to its intended spot, nurture it, and build the soil as the root zone expands over the years.
Bastian offers another stratagem: "Cross our fingers and hope we don't have another drought."