April is a very changeable month, but most of our temperate and cold-hardy trees can be taken outside now. This is the best time for re-potting: deciduous trees first, when the buds swell but before leaves form. Azaleas and Chinese Elms can still be re-potted. Conifers, including junipers, cryptomerias, cypresses, etc. can be worked on now. Pines can be done last. The slight set-back they suffer from root disturbance will help shorten their needles.
And it's already time for some pruning. Maples, which send out new leaves in bundles of three, can be pruned by removing the center shoot, which will help ramification and reduce the legginess of early growth. Deciduous trees whose branches have grown five or six new leaves can be pruned back to two or three. Azaleas should be pruned as soon as the flowers fade, and before they set seeds. Junipers can be finger-pruned, and the new tips of spruces and cedars can be shortened by a third to a half. Pine candles can be broken off just before they needle out - but have a plan, because this is the primary means of balancing the strength of the different areas of the tree. The Bonsai Today book Pines gives excellent advice.
It's still too early to leave tropicals outside at night, but they can be pruned, and they will benefit from outdoor sun, if you're up for the daily shuffle. Re-potting of most tropicals is best done in the longer days of summer.
As it gets warmer, some of our trees will benefit from midday shade. the sun's intensity is similar to what we get in mid-August. Warm days, too much sun and drying winds can all cause those brown leaf margins we dislike, as water is transpired from the leaves faster than the roots and circulatory system can replace it. Small-leafed trees, especially needle species like pine, cedars, and junipers can stand more sun and wind. Maples like partial shade. Watering needs careful attention now, as the weather can greatly affect the amount your trees may need. threes in full leaf and those that are root bound use more, while some in semi-dormancy use less. Never over water your pines. if we have a segment of soggy weather, consider propping them up to allow quicker drainage.
Air-layering and grafting are best accomplished at this time as is sowing the seeds of most species. Pests and diseases are coming back into season again, so you may need to initiate a spraying program to combat them. Good combination sprays are on the market - follow direction, and be sure they're OK to use on your particular trees. Systemics are also valuable. Wiring is possible on some trees, but allow those that have just been re-potted and/or root-pruned to have several weeks rest beforehand. trees with a lot of leaves, like maples, are difficult because you can't see what you're doing, and you'll usually cause some damage