How to plant a tree
Choosing a tree carefully and planting it skilfully will help to ensure that it has a long, healthy life as a major focal point in your garden.
Bigger does not necessarily mean better when it comes to trees. When advanced trees are transplanted, they may lose up to 90% of their functioning roots. It is little wonder that it can often take a long time for the tree to start growing again - sometimes years. Smaller plants are less likely to suffer such a high degree of transplant shock. They are easier to handle, and if they have been propagated well in suitable containers, they can go ahead very quickly and develop into strong, healthy trees with good root systems. Within five years there may be little difference in size between a small plant and an advanced specimen. In fact the small tree may well have outstripped the advanced one. However, sometimes we have to choose larger plants for aesthetic reasons or simply for visibility so that they are not so easily damaged by mowing, kids, etc. In this instance, look for trees that have had lower branches trimmed.
In the past, the thinking was that pruning a tree at transplanting time made it easier for the plant to cope with the loss of so many of its roots. Recent horticultural research has indicated that there is no scientific basis for this idea and in fact, the loss of leaves reduces the plant’s ability to photosynthesise and may make it more difficult for it to develop the vital new roots required.
The Planting Hole
The most important thing is to dig a really good planting hole. Bad holes are too narrow, too deep (or both), round and smooth sided. A good planting hole is dish-shaped, much wider than the width of the plant’s root ball - at least three times as wide and more if the surrounding soil is compacted. It should be no deeper than the depth of the existing rootball. You may loosen the soil below with a fork, but don’t dig it out. Don’t put compost or manure in the bottom of the planting hole - anaerobic gases may be produced which will harm the roots of the plant. Do not add fertiliser to the bottom of the planting hole either. Roughen up the sides of the hole so that roots can easily enter the surrounding soil. Smooth sides can resemble the sides of a pot and can cause root circling which can severely limit the growth of the tree.
Planting the Tree
The most important thing to remember is to treat the roots of the tree with great care. Don’t leave the pot standing in the hot sun while you dig the hole. Don’t lift trees by their trunks. Don’t pull at roots to "tease them out"! If you find there are roots circling around in the pot, take a sharp knife and score to about 5mm deep down the sides of the root ball in three different places. (Reject any tree that has doubled back roots near the trunk. This is a sign of poor propagation techniques and the tree will almost invariably be short-lived or snap off at the base at some stage.)
Place the tree in the planting hole. Be sure that the top of the potting medium is not below the level of the surrounding soil. If you wish to add organic material to the backfill only add a small amount of well-composted material (about 5% of the volume) and mix it in well with the site soil before back filling. Make sure there are no air pockets, but do NOT do this by stamping on the soil which will cause compaction. It is very important that the top of the potting medium is not covered by the back fill.
Diligent watering during the establishment period of the tree is the single most important success factor. With limited roots, the tree cannot be allowed to dry out. This applies to all species, including drought resistant ones. They will be drought resistant only after their roots are well-established. Initially they need watering. This TLC should continue throughout the first year. There is no credence in the idea that you can "toughen up" a plant by treating it badly - you will simply end up with a plant of lesser quality (or no plant at all). Watering should consist of deep soaking , not just light sprinkling. You can build up a small ridge or berm just beyond the root ball to encourage moisture penetration if you wish. Many trees will not cope with having large amounts of soil piled up over their root zone, so construct any garden beds, rockeries etc that are to be below the canopy of the tree (or near by) before you plant it.
Do not fertilise a tree at planting time. Instead, water it well until you see signs of shoot growth. Then you can fertilise. A slow release fertiliser is a good choice. Remember to place fertiliser away from the trunk - put it in the area where the new roots will be making their way into the site soil. Remember that the most important roots are in the top 15cm of the soil where there is adequate oxygen supply.
Mulch is important for vital moisture retention and to reduce weed competition. Don’t apply mulch too thickly (75mm or 3 inches is the maximum for most mulches). If the mulch hasn’t been composted, then the fertiliser you use should be high in nitrogen. (Nitrogen is consumed by micro-organisms as they decompose organic material. This is called nitrogen draw-down) Keep mulch well away from the trunk of the tree to prevent fungal attack such as collar rot.