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Don't top your trees
The buds at the tips of shoots in trees are hormonally controlled. Those on lateral branches are controlled to grow outward, toward light (phototropic). Those on tree tops are controlled to grow upwards against gravity(geotropic).
When topping cuts are made and the geotropic buds are removed, the tree switches the hormonal signals to the buds at the tips of the upper lateral branches to start growing against gravity. Some species of trees will also trigger dormant buds that lay under the bark (epicormic) of the tree to release and grow geotropically, known as watersprouts.
Either type of newly recruited geotropic buds will grow many times faster than those of the original top, often quickly re-attaining the height of the original top. The result is a new top far larger and denser than the original. At this point, any benefit of the topping cut is undone and the original issue is more of a problem than before.... but it gets worse.
Trees are rarely able to close off wounds from topping cuts fast enough to stop decay fungi from entering the open area. As the newly recruited tops curve upward and put on rapid growth, the area around their point of attachment to the trunk becomes progressively more decayed inside, thus weakening the new tops' attachment, already weak because of the two 90-degree turns from the stem.
The obvious result of many large new tops with weak attachment points is a tree that can become very hazardous. Accordingly, many of our municipalities prohibit topping in their tree bylaws, unless it is for hazard reduction in previously topped trees.
Don't pile soil or other debris over your tree's root system
Tree roots require air to survive. On the other hand, too much air will desiccate roots and kill them. As roots grow outward, they achieve a delicate balance of air, moisture, and nutrient supply in the soil, with the fine absorbing roots usually being found in the top few inches.
When extra soil is piled over a tree's rooting area, it decreases the amount of oxygen that can get to these roots, often killing them. This also renders the lower soil environment more favourable to root decay fungi which, in serious infections, can cause the entire tree to fall over. Adding as little as two inches of soil to the rooting area can be enough to kill a mature tree.
Don't over prune your trees
While some orchardists rely on heavy pruning to maintain a heavy fruit yield, this is not a suitable practice for most homeowners. Orchard trees are grown specifically for fruit production and not for aesthetic value. Such pruning practices severely reduce a tree's life expectancy, compromise structural form, and come at heavy maintenance cost due to excessive sprouting. Heavy fruit crops are a symptom of stress, and such trees are pruned so as to stress them.
Trees grown for ornamental value, as is the case in most yards should normally not have more than 25% of the leafy area removed in a year. Keeping within such limits, while making proper cuts, can keep a tree healthy and maintain an attractive form. This can be done while, at the same time, maintaining a decent yield of fruit, if so desired. Pruning is all about balance.
Don't fertilize unless you know your tree needs it
Any nutrient becomes toxic when there is too much of it in the soil. Because trees are much longer-lived than most other garden plants, they keep a delicate equilibrium between growth rates and other physiological functions and moisture and nutrient levels in their environment.
Upsetting this balance by adding concentrated nutrients can have unintended and unwanted growth reactions in trees. If a tree shows symptoms of nutrient deficiency, it is best to have the soil tested before trying to amend the soil.
If fertilizer is required, it is best to use an organic form, in which nutrients are released at a slower rate and are therefore less likely to cause fertilizer burn than a synthetic variety. The easiest way to gently amend your soil, is to allow a tree to take back its own nutrients, by allowing leaves to compost themselves on-site.
Don't over water your trees
While trees do need water to survive, too much water can kill them. Roots need access to air through soil pores, which can be cut off by oversaturated soil. Roots in oversaturated soil will eventually die. Wet conditions are also very favourable to many of the fungi responsible for tree decay, especially for root rot fungi, which can result in the entire tree falling over.
Don't get in over your head
If you are considering working on your trees and something seems unsafe, it probably is. There is a lot of weight involved and a lot of forces at play even in fairly small trees, and many homeowners are seriously injured or worse yearly when trying to do work they are improperly equipped for. If unsure, call an expert.
If you are considering working on your trees and aren't familiar with proper pruning techniques, trees' growth responses to pruning or damage, or the needs and limitations of the particular species of tree in question, it is wise to call an expert.
Short-term savings on doing work one's self are often dwarfed by the long-tem cost of attempting to repair a tree from improper pruning.
Do know the species of your trees, and the personalities of those species
Each species of tree has its own specific needs and habits. Knowing these ahead of time can help you avoid actions that will harm your tree, or prevent you from planting the right tree in the wrong spot in the first place. Western redcedar (Thuja plicata) has a shallow, spreading root system that allows its roots to survive the wet environments it grows in naturally.
This species would be inappropriate for a fast-draining hilltop, or a site where heavy traffic is expected over the rooting area. Most birches (Betula spp.) are likewise adapted to wetter growing sites, often naturally growing along creeks or in gullies.
This is one reason we see a lot of otherwise beautiful birch trees in Vancouver planted in fast-draining lawns with dead tops. Pacific dogwood (Cornus nuttallii), the bearer of our provincial flower, is very sensitive to damp conditions and stagnant airflow, and will often quickly die in such conditions from dogwood anthracnose.
Knowing the average mature size and spread of a tree is of utmost importance in choosing its planting location. Every year, we are called to remove otherwise beautiful, healthy trees, that have been planted in the wrong spot and are damaging buildings.
Do plant trees in appropriate spots
This goes along with knowing your species... We are surrounded by temperate rainforest full of beautiful trees. Where there is room for them to grow, large native species such as Douglas-fir, western redcedar, western hemlock, grand fir, and bigleaf maple can be marvellous assets to a yard. The same goes with stately exotic trees such as black walnut, beech, or elm. However, property owners frequently don't take mature form into consideration when planting seedlings, or allowing naturally seeded trees to grow in place.
A bit of planning while a tree is small can reduce the need for expensive removals down the road, along with the loss of an otherwise beautiful tree. Trees that grow large also tend to grow surprisingly rapidly.
Do water your trees if they need it
Trees growing in our region have acclimatized to our rainy environment. Established trees should have no problem dealing with a week or two of drought when we do get dry breaks. However, prolonged droughts can unnecessarily stress your trees, reducing their vigour and thereby reducing their resistance to pathogens.
Trees benefit most from infrequent (no more than twice a week) deep watering than from regular short bursts with a sprinkler. They do not like their trunks being sprayed directly by a sprinkler, rather a gentle soaking of the soil around the root area, ideally with a soaker hose. Watering is most effective in the evenings, as during the day much of the water is effectively lost to evaporation before roots can absorb it.
Do have limbs pruned back from your house
It is beneficial to prune tree limbs growing towards walls and roofs early on to establish a form that will require little maintenance in the future, and provide adequate clearance from buildings. Branches coming within 3 ft. of roofs or eves invite squirrels and rats onto rooftops, where they can gain access inside through roof vents, often nesting in attic space. This is a common problem in the Greater Vancouver area.
Dense branches over roofs and walls also limit airflow and promote a moist environment for moss and lichen to grow. Judicious pruning can promote increased airflow to keep walls and roofs clean, while maintaining the healthy form of trees.