Ten Rules for the Health of Old Trees by: Ruth S. Foster Why do certain trees live to be centuries old? Why have certain trees survived when the average life span of a tree is a mere 40 years? Three factors are probably most important. First is their generic makeup; some species are long-lived, others are not Second, these giants probably have tapped into a reliable underground water source. Third, they were not abused by man or beast. Farming, foraging and wildlife were the main hazards of the past. Construction damage is the main risk today. Keeping old trees growing for another century requires understanding what care they need, as well as what not to do. Old trees grow very slowly, just enough to survive. When they're abused or seriously stressed, they go into decline, and die. Old trees are like old people: fragile, but, with gentle care, can survive a while more. 1. Be conservative. Don't change the habitat where the tree has been doing well. 2. Treat only problems that are important and threaten the tree's health. All old trees have a few bugs and a little fungus, branches that break off and sometimes wetwood. These don't necessarily have to be completely cured, just kept under control. Unfortunately, some landscape professionals try, in good faith, to apply the same rigorous criteria to old trees that are correct for younger ones. 3. Don't push fertilizer and force an old tree to grow faster. One wants to keep it alive, not make it bigger. Only fertilizer that will enhance health should be used. And that, sparingly. The standard advice is no more than every 3 to 5 years. 4. One of the biggest mistakes is to overprune. Certain old trees, particularly beech, oak and sugar maple die back and can even be killed by sunburn on the highest branches and trunk. Pruning should be minimal on old trees, only to remove dead or dangerous branches and keep the tree well-balanced. Leaves can upper bark shouldn't be suddenly exposed to sunlight. 5. Don't change the root environment under a healthy tree. This is the most important thing. The roots of old trees spread at least twice as wide as their leaf canopy. Most tree roots, particularly beeches, require mychorrizhea, microbes and other mysterious microscopic organisms to thrive. Grass competes with and inhibits these organisms. A wide bed of mulch under trees enhances health. Under an old giant, ideally a bed with fallen leaves should be left in place to self-compost. Ground cover can sometimes be established; pachysandra and vinca minor are good. But if the wide shallow tree roots take all the moisture and make growing grass or ground cover difficult, just give in. Instead, make a circle with wood chips. 6. If you want grass, be content with fescue, and reseed as often as necessary. It is not good to put sod under an old tree. Also remember, herbicides are absorbed by tree roots and are especially deadly to beech and locust. 7. Don't ever add any fill, even good loam, deeper than inches over the roots. Six inches of anything on the roots of a beech or a sugar maple can kill it. Even piling extra earth around the trunk can cause problems. 8. A new sprinkler system under the tree will disrupt the existing metabolism and may rot the roots. It is not a good idea. Beyond the edge of the leaf canopy, new roots will eventually find the new water, and new tree will gradually adjust. During summers of severe drought, soak the roots really deep when needed. Turn a hose on for 24 hours at each part. It may take several days to cover the whole root run. Then wait until things dry out again. If each soaking is adequate, it is rarely necessary to do this more than twice a season. 9. Because the root run is so large, big trees can tolerate losing a small portion for necessary construction. However, large major roots should not be excavated but pipes and other construction items can be pushed underneath. 10. If the tree seems in trouble or has a problem, it doesn't hurt to get a second opinion. When old trees go into decline, things can be done to help delay the inevitable, often for many years.