Common Tree and Shrub Myths

Spring is just around the corner, and along with it – planting and growing season! In the spirit of the change of season, we pulled the following article from our reference library of landscaping topics. This time, we decided to share some useful information from one our favorite guide books, Gardening Myths, which includes some practical tips and best practices for fertilizing trees, and also the best time of year to plant. We highly recommend having books like these on hand, but in the meantime, enjoy this article and as always, let us know how we can help you with your tree service needs!…

Myth: Deep Root Fertilizing is Good for Trees

Deep root fertilization is a recommended procedure by many arborists, and equipment is available for DIY homeowners. Is it the best way to fertilize trees? Do they need to be fertilized?

In deep root fertilization for trees, you stick a pipe into the soil about 6″ to 12″ and then, under pressure, squirt fertilizer into the ground. The theory is that since tree roots are deep down in the ground, this method gets fertilizer to the tree roots quicker and other plants such as grass or perennials do not steal it away from trees.

Is it interesting that the International Society of Arboriculture (ISA) recognizes this as a gardening myth, and yet many ISA certified arborists still sell the service. According to the ISA, “You don’t need to perform “deep root fertilization” to reach their root system – most of the trees’ fibrous, absorbing roots are in the top eight inches of soil.” A recent review of available literature by Daniel K. Struve concluded that, “Little difference has been found among fertilizer application methods; broadcast applications are as effective as subsurface applications.”

Most absorbing tree roots are found int eh top 2″ to 8″ of soil where water and oxygen are abundant. Fertilizer placed below this level does little for the tree and is in fact environmentally harmful.

If you have healthy soil, you probably don’t need to fertilize. The problem in our urban landscapes is that we remove the tree leaves each fall. The leaves are nature’s way to fertilize the trees; by removing them you are removing the food for future years.

Grass growing on top of trees roots will compete for nutrients. Grass can be a problem if you need to apply high levels of nitrogen for the trees, since this will burn the grass.

Deep root fertilization can be done in the following ways:

  • By an arborist with special equipment that will inject a liquid fertilizer. If not done properly, this method will inject below the majority of roots. If it also expensive.
  • Homeowner injection equipment will not inject as deep as professional units, which is probably a good thing. This works, but the special fertilizer pucks designed for the equipment is more expensive than bagged fertilizer.
  • Holes can be drilled in the ground around the tree and filled with fertilizer. This provides high levels of nutrients in specific spots, but the fertilizer does not move radially very well, so some roots are over-fertilized and probably burned, while others get no fertilizer.
  • Fertilizer spikes can be hammered into the ground, but this has the same problem as drilling holes.

Best Practices

  • The least expensive and most effective way to fertilize trees is to broadcast granular fertilizer or organic matter on the ground. Nature will move the nutrients deeper into the root zone.
  • If you are fertilizing your flower beds and your lawn, you probably do not need to fertilize the trees.
  • The best way to know if you need to fertilize is to get a soil test done.

Myth: Fall is the Best Time to Plant Trees

What is the best time for planting a new tree or shrub? Many people plant in spring, probably because they want to do some gardening at that time of year and garden centers have the best selection. Others claim that fall planting is better.

Landscapers should know the best time for planting trees, so it makes sense to look at what they do Around here, zone 5, companies are planting trees most of the time, except in the dead of winter. I think this has more to do with the need to complete projects than a concern over plant health.

This past summer, landscapers planted trees at a local grocery store on the hottest day of the year. The next day, the shrubs looked like they were dead. They have since come back, mostly because they are a very hardy variety of dogwood. The middle of summer is not a good time to plant anything.

In warm climates, where the ground does not freeze, you can plant all year long, provided regular water is available. In cold climates, where you do get frozen ground during part of the year, it is a bit trickier.

Early spring seems like a good time for planting trees. Nurseries have the biggest selection, and trees have a chance to grow all summer to get ready for winter. Soon after planting, leaves develop and they are able to make food all summer. Keep in mind that we only see what goes on above ground. When a tree is planted, it needs to make new roots. Planting in spring means that the tree needs to make new roots at the same time as it makes new leaves. Both of these growth processes require sugar reserves that are stored in the roots and stems. Trying to grow both is taxing for the plant, and both processes suffer. The lack of roots also means that less water is absorbed from the soil, exasperating the problem.

Many experts claim that planting trees in the fall is better, since the tree can make new roots without having to feed the leaves. Water requirements are also lower without the leaves on the tree. To us it feels cool in the fall, but that is actually the best temperature for root growth. Roots grow best in cool soil and stop growing when it gets too hot. Fall planting gives the tree more time to develop roots before it needs to grow leaves in spring.

What about plant availability? There is probably less selection in fall, but good nurseries do have many plants available because fall planting is becoming more popular. The other benefit is that a of plants are on sale in fall, but a low price is only a good deal if the plant was cared for properly all summer. Some plants available in fall have suffered all summer from lack of water.

Fall planting allows tress to grow more roots before they need to make leaves, but do they survive the winter better than spring-planted trees? This question does not have an easy answer, but I suspect winter death is dependent on how the tree was treated before it was planted. Did it sit around for months and months in a nursery with not enough water? Has it grown a lot of roots in the pot and is now root bound? How much damage was done to the roots when it was dug up, and when it was planted? All of these factors affect survival.

According to Purdue University, some plants are more susceptible to winter injury from fall planting. Magnolia, dogwood, tulip tree, sweet gum, red maple, birch, hawthorn, poplar, cherry, plum, and many of the oaks are among those that do better with spring planting.

Fall planting of deciduous trees works better because of the lower water requirement in winter. Evergreens keep their leaves/needles all winter, and even though their metabolism is reduced, they still lost water all winter long. For this reason, fall planting is more difficult for evergreens, especially broad-leafed evergreens.

The best time for planting is also dependent on the gardener. For fall planting to be successful, the tree needs adequate water right up to the time when the ground freezes. If you are never in the garden in fall and forget to water, you are probably better off planting in spring when you are more likely to take care of the tree.

If you are still with me, you are probably wondering if spring might be a better time. Although many references are quite clear about which they recommend, the answer is not as clear as they make it out to be. The question was recently asked on The Garden Professors Facebook group, and the answer was a bit surprising. The Garden Professors are experts in trees and do tree and plant-related research (quotes are from the link listed below).

Bert Cregg, from the Midwest, says, “Location makes a difference. Here in the upper Midwest, my rule of thumb is to wait until spring unless there is a compelling reason that you need to fall plant. I do a lot of post-mortems on landscape planting failures and see proportionately more problems with fall planting than spring planting. This doesn’t mean fall planting can’t or doesn’t work; just the odds of success are better in the spring.”

Linda Chalker-Scott says, “As Bert says, location location location. The post-mortems I do often stem from spring planting and no irrigation. Fall planting in our climate (Washington State) is best (because of dry summers), and I’d argue it would work anywhere as long as the soil is well mulched to prevent freezing.”

The key point here is that trees need to have water during their recovery period of which the first six months is crucial. The west coast has a very dry summer, but a warmer and shorter winter, which is ideal for root growth. Hot summers are not. In the Midwest, summers are cooler and wetter, which are ideal for spring planting. Fall is shorter, and winter is longer and much colder – a difficult time for a newly planted tree.

Best Practices

  • For climates with longer, more severe winters, try to plant trees in spring. In climates with milder, shorter winters, fall planting works well. In either case, the trees must have adequate water, and be planted correctly.
  • Summer planting will work, but it is much harder on the tree and is not recommended.
  • No matter when you plant, mulch trees with at least 5 inches of wood chips and keep them well watered.

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