The Right Time to Prune Spring Flowering Shrubs

Among plants that may be pruned after flowering in spring are forsythia, beauty bush, shrub honeysuckle, viburnum, weigela, spirea, lilac and deutzia. An exception to this type of pruning is flowering quince. It flowers on two-year old wood. Heavy late spring prunings reduce flowering. Light prunings during the dormant period are most satisfactory. Most other flowering shrubs develop their flower buds on new growth during the summer.

When pruning, try to keep the plant in a normal growth form, rather than a globe or ball shape. Shearing plants may be suitable for formal hedges but it often reduces flowering and stimulates development of dense, weak shoots. Prune a little each year. It is difficult and frustrating to prune a large shrub that has been unpruned for many years. The first stage of pruning includes removal of any dead branches, or branches that are very thin and tend to droop downward more than normal.

However, if the plant normally has a drooping habit, try to preserve this character during pruning. There are two general techniques used in pruning shrubs. These are "heading back," and "thinning out." Both techniques normally are necessary to achieve best form and reduce size. Forsythias are a good plant to practice pruning on, since its wood cuts easily and growth is generally easy to handle. If plants are large, some of the oldest stems should often be removed by cutting out near ground level. This often provides adequate thinning.

Next, to reduce size, cut back long branches to healthy side branches. Cut out brushy growth that may have developed from previous improper prunings. Forsythia plants flower poorly when sheared as a hedge. Shrub honeysuckles may be handled in about the same way, but they more freely produce abundant basal shoots that must be thinned. If plants are healthy, bush honeysuckles may even be cut back near ground level with good recovery. However, if this is done, a thick growth of basal shoots usually develops and should be thinned out.

Spireas should be pruned immediately after flowering. Cut back flowering wood to strong buds or vigorous shoots. Allow some of the strongest basal shoots to remain, and take out their tips to make them form lateral flowering branches. Selection of the most vigorous basal shoots is also important in many flowering shrubs. Beauty bush normally flowers best on shoots growing from older wood. Therefore, pruning should never be done too severely at any one time, or flowering may be lost for one or more years.

Viburnums seldom need much pruning, but if they have become too large, cuts may be made back to strong buds or non-flowering lateral branches. Remove flowers from lilacs after blooming. Suckers should be removed from the base of grafted plants. Severe pruning in lilacs should be done gradually, reducing the plant size by cutting out no more than one third of the old wood each year.

http://cmg.colostate.edu/gardennotes/619.pdf