Periploca.—A climbing genus which should be thinned out in winter, and only shortened back if necessary.
Pernettya.—These should not be pruned at any time.
Philadelphus.—These should be thinned after flowering, and the old wood cut back to strong young shoots. This is especially important with P. microphyllus, P. coronarius, and P. Lemoinei and its varieties.
Phillyræa decora (Vilmoriniana).—This well-known shrub needs no pruning.
Photinia.—Requires no pruning.
Pieris.—Remove seed-pods.
Potentilla.—Thin out after flowering, and shorten any old wood back to strong young breaks.
Prunus.—When young, all the members of this genus that are grafted or budded are improved by being cut back each spring until they have attained a fair size and shape. More especially is this the case with the Almonds, double-flowered Peaches, and the various flowering Cherries. When older, they need only be thinned and the flowering Plums and Cherries kept spurred in, but not too hard. Prunus japonica, P. nana, and P. triloba should be cut down to strong young breaks after flowering, the resulting wood bearing better flowers than the old wood. If any of these three latter are grown on a wall they should be spurred back hard after blooming.
Ptelea.—When young, trim these to form small trees, and do not allow them to develop into ungainly bushes. When older, they require an occasional thinning. P. trifoliata var. aurea, a golden form which is not grown so much as it deserves to be, should be cut back annually or biennially, the young wood being better coloured and bearing larger leaves than the old.
Pyrus.—The wild Pears should be spurred in the same manner as adopted for fruiting Pears, though not quite so hard. The wild Crab-apples, such as P. baccata, P. floribunda, P. spectabilis, &c., should be cut back every spring until they have formed well-balanced heads. Afterwards an annual thinning and a shortening of the longest shoots after flowering is sufficient. The remaining sections of Pyrus merely require an occasional thinning. P. japonica should be kept spurred in, whether growing on a wall or in the open, and in the latter case should not be allowed to become a mass of weakly shoots.