Why Landscape Trees Need Fertilizer

I received a letter from a reader early last fall who had a very simple and understandable question: “Why do trees have to be fed?”

After all, long before man and woman came along there was no guy driving around in a white and green truck who injected fertilizer into every tree’s root zone, and Job hadn’t even made it into the Bible, let alone invented tree spikes.

Actually, the tree story is very similar to the fish story. That’s the one where a tiny fish becomes the meal of the slightly larger fish who is eaten by the slightly larger fish and so on and so on. At some point long before anything remotely similar to man roamed the Earth there were plants tinier than the fish. In time these plants would die, and their remains became the food for the next, larger plant — in a sense, composting, and to some degree, in perpetuity.

Then one day along comes this gal; we’ll call her Eve. Now Eve was out walking when she discovered this fascinating tree. Not just any old tree, but a tree with some round, red appendages hanging from its branches. Being a curious sort she pulled one of these things off a branch and smelled it, and it had a great aroma compared to some of the other things she’d found in the woods. Well, if it smells good it must taste good, she reasoned, so she took a bite. It was rather tasteless and mealy, but she thought she had something of great potential value.

Eve went home and thought some. She’d heard from the locals that some joker had planted fish heads around this olive tree in a kind of religious ritual and that miraculously the olive tree doubled in size and the most perfect olives appeared up and down every single branch.

Well, Eve went back to her tree, which she had since named Apple, and tried the fish trick. The first documented fertilizer was given to a tree, and the results were astonishing. Eve couldn’t believe the taste of the apples that fall, and she gleefully brought one home to Adam. The story takes a nonhorticultural turn from there.

It was through this little-known event in history that we began to understand that feeding trees can enhance their fruit and flower production, stimulate root and shoot growth, ward off infection from diseases, withstand the ravages of winter and summer, and help to make it through times of drought and stress. In more recent times we have learned that we have to feed trees so that they can survive the ravages of man.

Mother Nature didn’t put trees in potted planters, nor did she plant them along the median of paved highways or in small squares cut out of concrete sidewalks and curbs. The worst scenario was when Western man decided that he needed lush green lawns surrounding his housing developments, sub-divisions, and Hamptons estates and that trees should dapple the landscape. The lawns robbed the trees of any of the nutrients provided by Mother Nature through the natural process that allowed the nutrients to pass through the soil to the tree roots. The lawn got them all first.

For all of these reasons and others, we need to feed many of our ornamental and fruit-bearing trees in the landscape. Newly established trees will grow more rapidly following fertilization with a nutrient or nutrient combinations, which are available in only small amounts in our soils.

This is made clear the following season (if fall fertilizing takes place) by increased leaf size, length of current season twig growth and a much more rapid increase in height. Even among slow-growing tree species, many of which have desirable characteristics, feeding may stimulate faster and fuller growth. One can overdo it though, and an overfed tree can actually be weakened, stretched and more susceptible to insects.

Leaf color and leaf size often indicate nutrient deficiencies in the soil. Various colors or patterns of color indicate deficiencies of specific essential nutrients. Symptoms include one or more of the following: pale green or yellow color, leaves with mottled patterns between veins or with dead spots, stunted leaves and early loss of leaves — assuming that these are not the natural characteristics of the particular tree. The leaves of many trees become noticeably darker after feeding, making them more conspicuous and attractive.

Fertilizing can also help maintain mature trees in a vigorous growing condition, and this is most important in situations where man becomes the problem because of the ornamental or recreational use of the land near or under the tree, depriving or shorting the tree of the necessary soil nutrients. Lawns growing right up to the trunks of trees instead of being clear of the drip zone is a prime example as is the constant removal of falling leaves and twigs, which would otherwise decay to organic nutrients.

A vigorously growing tree is less susceptible to certain diseases and insect pests than a less vigorous tree. Canker-causing fungi occur more commonly on weakened trees, and many of the noninfectious tree diseases develop when soil nutrient and moisture conditions are unfavorable. Healthy, vigorous trees tend to resist borers while those growing under unfavorable moisture or nutrient conditions are more susceptible to attack by these insects.

Established trees that are weakened by disease, insect defoliation, mechanical injury, soil compaction (possibly from pool, tennis court or driveway construction), drought or other causes often show poor growth that is indicated by dying of branch ends (dieback) or minuscule annual growth. In most cases the use of fertilizers stimulates additional growth so that the plant can compensate for the conditions that cause decline.

I saw a dramatic example of this several years ago. A long driveway leading up to an estate home had 60 Kwanzan cherry trees planted on either side of the drive. The trees were installed shortly after the driveway was constructed. Because of nearby retaining walls the root zone for the cherries was substantially restricted so the small root system on the large trees couldn’t move far enough or deep enough to find enough soil nutrients.

For all this time, the trees put on maybe an inch of new growth every year — until several years later. That fall the trees were fertilized with a method called deep root injection. For the two years after that, the trees put on an average of 5 inches of new growth each year, and the stress that the trees were under, manifested by very early leaf drop, stopped. For nonbelievers who may have a privet hedge or two, feed your hedge lightly during November and again in May and watch how much longer they stay lush and green well into the next winter, instead of defoliating in the fall.

Next week we’ll try to decide what time of the year is the best time to feed your trees because there are several theories, and we’ll look into five ways to feed your trees — some at great expense and a couple that will cost just a few cents per tree so long as you supply the labor, warm gloves and earmuffs. Keep growing.