Nuttallia.—The single species of this flowers in February, and is improved by a good thinning out of the old wood when blooming is past.
Olearia.—Requires no pruning.
Ononis.—O. rotundifolia should be cut down every winter, as it flowers on the young wood. The remaining species flower on the older wood, and need not be touched.
Osmanthus.—These should not be pruned unless a particular shape is desired, when the plants may be clipped with a pair of shears in spring.
Oxycoccus.—This is a small creeping genus allied to Vaccinium, and requires no pruning.
Oxydendron.—Remove seed-pods.
Paliurus.—This attains the dimensions of a small tree, and should be kept trimmed up for that purpose.
Parrotia.—Thin out in spring after the flowers are past.
Paulownia.—Keep to a single stem to a height of about 8 feet, and then allow it to branch. If used for sub-tropical bedding, it should be cut down to the ground every winter.
Peraphyllum.—The solitary hardy species of this should not be pruned or disturbed in any way if it can be avoided.