Trees planted in good soil which is protected by a thin layer of some sort of organic mulch (chopped leaves, pine needles, wood chips) do not require regular fertilization. The mulch materials decompose over time and provide essential nutrients to the soil below. Then the many tiny organisms that reside in the soil convert these nutrients into a form that the roots can take up into the tree. This is nature’s efficient system, and providing supplemental fertilizer is not normally necessary for otherwise healthy trees. In fact, these days, proper planting guidelines no longer recommend adding fertilizer at planting time to the loose soil that is returned to the planting hole to cover the roots of the new tree. Click here for instructions on planting your tree correctly. Maintaining a two or three inch layer of organic mulch on the soil under the newly planted tree at least until it is established, does the job. Click here for instructions on mulching trees correctly.
There are situations, however, when supplying fertilizer to trees is helpful. One is when trees are planted in poor soil. Another is when trees grow old. Fertilization provides extra nutrition to help them retain their vigor when they are more vulnerable to all the stresses from extreme weather, insect and disease attacks and damage from human activities.
Feeding Recently Planted Trees
Delay fertilizing for a year or two after planting to give the young tree a chance to adjust to the soil that it now lives in. Artificially boosting the nutrition of the planting soil may actually delay or distort root development and encourage rapid top growth that the roots cannot support. Enriching the planting soil also encourages young roots to grow in circles in their planting hole instead of spreading out into the soil beyond it. The exception to this guideline is situations where the soil is horribly deficient in nutrition. Adding some fertilizer to the planting soil at planting time will help get the young tree started growing roots. Fall, after the leaves have dropped, is the time to fertilize trees.
Once a young tree has become well established and its roots have spread properly out beyond the area that its leaf canopy covers (approximately 5 years), there is no need to routinely fertilize anymore. There may be certain situations where a tree is struggling and fertilizing is appropriate. Consult a certified arborist to help you determine this.
Choose a granular fertilizer labeled for trees and shrubs. It should also say “slow acting” or “slow release” on the label. This assures that the fertilizer releases the nutrients into the soil gradually, providing consistent and uniform nutrition over as many weeks as are indicated on the label. One fall application per year for a healthy tree is sufficient. Some fertilizers are also formulated particularly for trees that like acid soil, so it is important to know what species your tree is to make the correct choice.
Sprinkle the granular fertilizer evenly on the soil (or the mulch) over the area where the soil was disturbed at planting time and a bit beyond. Do not exceed the amount of fertilizer recommended on the label; in fact, you might use a little less. Plan to spread it when rain is expected and it will soak the fertilizer into the ground for you. Otherwise, water the fertilizer covered soil promptly. It will not work without moisture.
Feeding Older Trees
As a tree ages, it becomes less efficient at absorbing soil nutrients, creating energy by producing carbohydrates from the sun (photosynthesis) and pumping this energy through its system to fuel the creation of new leaves and maintain its immune system. It is sometimes advisable to fertilize older trees to boost their efficiency and vigor. Also, some trees sustain damage from storms or insect infestation and must undergo severe pruning. Again, fertilization may be advisable to help the tree heal its wounds and recover its vigor. In these situations it is best to rely on the advice of a consulting certified arborist as to which fertilizer and how much is appropriate.
It is easy to take the trees on our property for granted. Standing stolidly out there in the yard through all kinds of weather, they give every appearance of self-reliance. Even as we rush around during dry periods watering the annuals in the windowboxes, the tomato plants, and the drooping perennials and shrubs, we forget to water our trees. We assume that our trees can cope. It is a compliment to their incredible resilience that it is often the case that they do cope with a fair amount of neglect from us.
However there are times when trees truly need our help to maintain their optimum strength and vitality and to grow to their fullest potential. One critical time is when they are first planted. Planting a tree correctly gives it a good start. Generous watering gives it a good future. Another time is when a tree grows old. Watering helps prolong its life.
Water is critical to trees of all kinds. In fact, some, such as baldcyress and Atlantic white cedar don’t mind living in damp soil. All trees have a hydraulic system that is superb at pulling moisture out of the soil—one of the reasons that trees are so helpful in controlling flooding. Trees as Stormwater Controls. While a layer of organic mulch on the soil in the root area of the tree is most helpful in maintaining soil moisture, sometimes trees need supplemental water from us.
Watering Recently Planted Trees
At no time is water more crucial than when a tree is newly planted. Whether it is bareroot, balled and burlapped or containerized, once it is in the ground it needs lots of water on a regular basis either from rainfall or from you. For the first year or two, a young tree needs 5 to 10 gallons of water a week. It will need even more, during periods of serious heat or drought.
Older, established trees can usually manage pretty well on their own. However, if their soil is sandy, if there is excessive heat or drought, they, too, will benefit from supplemental watering. Remember, mature tree roots eventually spread out way beyond the extent of the tree’s branch canopy, typically from 12 to 18 inches below the soil surface, so they benefit from watering over a wide area. Any grass under your trees competes for water there, especially during drought, so spread mulch or plant shade-loving shrubs or groundcover plants under trees to help conserve soil moisture.
The best way to deliver water to a tree is to use a soaker hose, also called leaky pipe irrigation tubes (typically made from recycled automobile tires). Attach it to your regular hose and then strategically arrange the pipe on the soil or mulch under the tree that needs water. When you turn on your hose faucet, the tubes gently leak water. It penetrates the soil or mulch slowly over time so that the soil really absorbs it and there is very little water lost to evaporation or runoff. A couple of strategically placed 5 gallon pails with small holes punched into their bottoms, then filled with water that leaks slowly onto the soil work essentially the same way. Certainly hand watering from a watering can with a rose on the spout to create a spray effect directly on the soil will also do the job.
First, determine that the soil is dry and if watering is needed. If you have a houseplant moisture meter, stick the probe into the soil under the tree away from the trunk a bit. Note if the indicator is on the “dry” side of the meter. Another method is to insert a screwdriver into the soil. If it enters easily, the soil is still somewhat moist. It is extremely helpful to maintain a rain gauge in the yard somewhere. This helps you track rainfall and estimate when younger plants may begin to feel a moisture deficit.
Exactly how much water to provide is somewhat a function of the type of soil the tree is planted in. Clay soils delay the absorption of water into the ground, so water thoroughly but less often. Sandy soils let water soak in rapidly, so water more often, but give less water each session. To measure how much water a soaker hose is delivering, put an empty cat food or tuna can on the ground under the soaker hose at some point. The can is one inch deep, so watch how fast it fills up, and you can time your watering session to achieve one, two, three inches or whatever.
Watering Established and Older Trees
Established and older trees rarely need regular supplemental watering. Their wide-ranging roots are able to exploit a huge area for moisture. However, there are some circumstances when it is necessary to provide water for established trees.
- Trees that are planted on berms, especially if they are not mulched.
- Trees that are not mulched and/or grow in compacted soils.
- Trees growing in regions where drought has become chronic.
- Trees that have been recently dug up and re-planted at a new site.
- Trees that lean or have been knocked over by weather or trauma and then righted, repositioned and replanted.
- Trees that are accustomed to some shade that now grow in direct sun due to loss of nearby trees.
- Trees that have endured severe pruning.