I planted a clump of three birch trees. Do the trunks need to be trained to get the desired spacing or angle as they grow?
This is more of an esthetic issue. Only you know what the right look is. If this was a bonsai you would be justified in any amount of wiring, wrapping and pruning to achieve one of the many styles.
For outdoor plants beauty is in the eye of the beholder. The less staking and wiring you have to do to a tree the better. As this ISA publication clearly states "Stake or guy a tree only when necessary for the specific conditions encountered."
You may wish to do corrective pruning for broken or crossing branches or if the canopy is very thick at the expense of foliage at the lower levels. My experience with birches is that they cast a light shade and do not tend to get too thick.
What is more likely to be an issue in urban areas is the birch leafminer and the bronze birch borer. Between the two of these many birches in urban areas have died and it is almost impossible to grow European or Asian birches in North America due to them.
Long term success with birches is enhanced by planting them in an area that mimics their natural environment on flood plains or close to water. The dry, compacted soils with competition from grass that are typical of urban areas are not a good fit.
Some ideas that may help:
- if the area is dry divert downspout water or water during drought conditions
- remove all grass for at least a three foot ( one meter) diameter from the base of the trunk
- top dress from the trunk to the drip line twice yearly with compost or similar to enhance the organic matter of the soil
- do not plant near a heavy traffic are. Compacted soil is hard on trees.
- observe similar birches in your neighborhood. Do you see birches dying from the top downwards in a single season? If so, the leaf miner and borer are present in your area and might attack your birch depending on species and stress levels.