Why Trees Need Fertilizer

I’ve told this tale before, but I think it needs retelling.I received a letter from a reader, many autumns ago, who had a very simple and understandable question:

“Why do trees have to be fed?”

After all, long before humankind 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 fertilizer 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, which 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 woman; 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 small 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 suddenly doubled in size and the most perfect olives appeared up and down every single branch the following season.

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 followed, and she gleefully brought one home to Adam.

The story takes a non-horticultural 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 some but not all 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 medians of paved highways, nor in small square cutouts of sidewalks and curbs. The worst scenario was when Western Man decided that he needed lush, green lawns surrounding his housing developments and subdivisions, 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 sucked them up first.

For all of these reasons, we need to feed many of our ornamental and fruit-bearing trees in the landscape. Newly established (but not newly planted) trees will grow more rapidly following fertilization with a nutrient or nutrient combinations that 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 become more susceptible to insects.

About 30 years ago, when the woolly adelgid began to decimate our hemlocks, the knee-jerk response was to feed them to help them fight this insect. So we fed them—the hemlocks got lush and green, and all this lush growth was like candy for the adelgids, and we made the problem much, much worse.

Leaf color and leaf size often indicate nutrient deficiencies in the soil. Various colors or patterns of color indicate deficiencies of specific essential nutrients, and a good arborist will pick up on these signals. 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 also can 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, recreational or commercial 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 pests than is 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’s indicated by dying of branch ends (dieback) or minuscule annual branch 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 on an estate several years ago. A long driveway leading up to the 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. The small root system on the large trees couldn’t move far enough or deep enough to find enough soil nutrients. For many years, the trees put on maybe an inch of new growth every year, until a great arborist did some serious thinking.

That fall, the trees were fertilized using 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 non-believers who may have a privet hedge or two: Feed your hedge lightly at the drip line during early 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!