Lottah Nursery Tasmania, Australia

LILACS - PRUNING

Lilacs perform quite satisfactorily in their natural habitat without any pruning; species under cultivation do not generally require much attention.
The hybrids and selections made for larger flower trusses usually benefit from some pruning to improve flower quality as the plants mature.

Pruning S. vulgaris and S. x hyacinthiflora hybrids
Space permitting, these cultivars are best grown with multiple stems to enable renewal pruning on a regular basis when flowering capacity is past their prime. Somewhere from 6-12 shoots are optimal.
There is probably no necessity to prune in the early years apart from removing competing shoots. As the plants mature remove 10-20 percent of the main stems annually so that there will always be no older than 5-6 year old main stems to produce the best flowers. Reduce overcrowing in the centre to encourage air circulation and let in the light.
After flowering the spent heads require removal to prevent seed set which will divert energy that should go into more vigorous growth and flower initiation for the following season.
Is there a 'correct' way of pruning these lilacs? Not that we know of. Many means will achieve the same ends, which is large quantities of high quality flowers. Plate 43 image of 'Cheyenne' in Fiala's book is fairly close to our ideal lilac shrub.

Pruning S. x 'Lotta' (and other repeat flowering cultivars)
Because of the repeat flowering characteristic this hybrid needs to be treated differently. More experienced growers suggest shearing the plant lightly after flowering to enable fresh growth for the secondary flowering; in any case the spent flowers need removal after they peak.
We feel that removal of some of the mature stems in Winter would encourage vigorous new growth for the best flowers, even if this means loss of some flowering capacity the following Spring.

S. x prestoniae
These hybrids do not generally sucker so renewal pruning of main trunks will not be possible. They are best grown as shrubs or trees with multiple trunks; only corrective pruning is needed for shaping and to reduce crowding.

S. reticulata
reticulatas grow as trees in their natural habitat; under cultivation they may be grown as shrubs or as trees; shaping and corrective pruning in the formative years is probably all they require.

Dwarf forms
The smaller lilacs do not generally require pruning although doing so might encourage better flowering from vigorous new growth.

External Link: More on pruning