Buy young trees - they are more vigorous than older trees and will recover faster from transplanting, and soon settle down and grow quickly. If you want to plant at least 0.25 ha, you may be able to apply for money from the Woodland Grant Scheme - contact the Forestry Authority for details. The best time is November, so the trees have a winter to settle in before the growing season. March - May. Plant your trees as soon as possible after they arrive. The largest size you would usually need to plant, is a 150 - 180 cm tree known as a whip.
Larger trees cost a lot more and can be a lot of bother.
Better to buy five whips for the same price - even if four die you've still got a tree. Pesticides can be used or, if you prefer, a mulch or mulch mat. A mulch mat is a mat placed around the tree to prevent the growth of weeds by keeping the light from them. Mats can be quite cheap, and you can make your own from reclaimed material such as old carpet or plastic sheeting. Water trees throughout their first two or three growing seasons, until they get established.