As gardeners, we worry about and provide for the nutritional needs of our flowers, vegetables and lawns-after all, these plants grow palpably before our very eyes. Trees, on the other hand, develop more slowly. We are more likely to notice their spring blossoms or vivid autumn leaf colours than their growth rates. Nevertheless, their apperance, performance and overall health can be improved with regular booster shots of fertilizer.
Young deciduous trees (three to 15 years old) have a more accelerated growth rate than mature ones and require annual feeding. Older trees and evergreens that are ade-quately irrigated (and therefore not under stress) will benefit from a feeding every second year.
When to Fertilize
Late spring or early summer is the best time to feed your trees. That's when they're actively growing and able to use the nutrients most effectively. Theoretically, trees can be fertilized up to eight weeks before leaf drop in the fall, but applications made late in the season may stimulate new growth that won't have a chance to harden off before freeze-up. This late growth inevitably dies back over the winter, leaving the tree susceptible to disease and insect infestation.
The Grass is Greener
Avoid using lawn fertilizer within one to two metres of the outer edge of a tree's leaf canopy: many contain herbicides that can damage trees. Their formulas also have much more nitrogen (the first number on the label) than most trees require. (Excess nitrogen promotes lush, green growth on flowering trees and shrubs-such as crab-apples and lilacs-at the expense of blooms.)
Hit the Spot
To be most effective, fertilizer must come into contact with a tree's small “feeder roots,” which are located at the dripline and extend out at least one metre beyond its perimeter, but just 15 to 45 centimetres below the soil surface. While deep, woody roots provide stability, only the feeder roots are capable of taking up water and nutrients.