This little article talks about the pruning of evergreens, perennial and deciduous plants and trees not specifically oriented to bonsais but to plants in general. It is not supposed to be a complete reference but a brief general guideline with some hints on special species.
Evergreens should be pruned when their growth is at its minimum, that is when temperature drops down to 4 or 5 degrees Celsius. In Madrid this normally occurs in november, so mid-november is a good moment to apply a intense pruning to give them back their wished shape. To prune once or twice a year should be sufficiant, but some trimming may be needed to keep it in shape. If the growth of the tree is very intense, as could happen with young plants, another good moment to prune is in early spring, when the tree is starting its most intense growth.
In the winter section of my garden you can see an image of the arizonica hegde in my garden freshly pruned.
Deciduous plants can normally be pruned at anytime during the year, but keep in mind that flowering shrubs should be always pruned right after bloom time. Shrubs like Privet or Dogwood can be pruned at anytime, but the best thing is to wait until they have lost the totality of the foliage, it is at that moment the plants stop to pump sap to the branches and the best moment to prune. At this moment it is also easier to give trees their shape, especially for shading purposes, as branch structure is visible during this time. In any case, only too long and out of shape branches should be cut.
Perennials, in general, do not need to be pruned necessarily if it is not required to keep the plant in a special shape. But the plant should be kept clean of old leaves and dry branches, work which should be done in spring.
Some hints on special species:
Grasses: The growth starts in springtime, so it is a good moment to cut off old growth.
Flowering trees: ie. Magnolias are normally trimmed after flowering, others can be trimmerd most anytime, but after blooming is always the best advice. Shrubs like Forsythia or Weigela also need pruning right after bloom time. Wisteria floribunda should be pruned in late fall to increase flowering in spring. Hydrangea, prune in early spring when new leaves appear on last years wood. Potentilla and Spirea should be done in early spring.
As special interest for bonsais, rhododendrons and azaleas should always be trimmed right after blooming, this normally happens in june. If they are trimmed to late this circunstance could cause less or no flowers for the following year.
Fruit trees: should be trimmed in late winter or early spring. They have no leaves then and branch structure is visible. The idea is to thin out the branches to get more sun to the growing fruit. It is also very important to apply oil spray before the leaves grow, to avoid rottening of the fruit.
Yuccas: When the yucca flowers, the rosette dies. The flower stem should be cut as it browns in late summer and wait until the following spring to cut off the dead rosette down at the ground.