Ornamental trees will take some years to reach the size at which they can support themselves. Until then they should always be adequately staked. You must check the stake regularly for rotting and replace it immediately if necessary. At the same time, make sure no parts of the tree have started to rub on it. If you do see this happening, wrap the branch where it touches with hessian or old nylon tights. Also check the ties, and loosen them if they are becoming rather tight. They should not be allowed to cut into the bark at any time.
Once trees, shrubs and conifers have become established for more than a year, you can eliminate weeding to a large extent by using a suitable weedkiller such as Casoron G (dichlobenil). Applied in spring, it should remain active, if the surface of the soil is undisturbed, for up to twelve months. Do make sure you follow the instructions, especially for any shrubs you should not use it near. If you suspect any plants are growing less well, you can refrain from using weedkiller for a year or two. You will get fewer weeds anyway if you mulch regularly. You cannot use total weedkillers if you intend to interplant with bulbs, annuals or herbaceous perennials.
Once they have been established for a few years, most trees, shrubs, etc. will not require watering in the same way as annuals, herbaceous perennials and the like, as they have wide-spreading roots capable of seeking out moisture in the soil.
An annual top-dressing of a slow-release, balanced fertilizer in spring is beneficial.