Gardening Australia - May 2008 Sample

Let’s look at lilacs. We call them lilac trees, but in most parts of Australia they are really just big, round-headed deciduous shrubs, most of which seldom grow more than 4m tall with a spread of about the same.

We grow them in our gardens for their beautiful, sweetly fragrant flower trusses, carried in mid to late spring in a range of colours including white, violet, blue, lilac, pink, magenta and purple. The flowers are clustered in showy panicles that should be upright, but tend to droop with the weight of the densely packed, waxy blooms.

The technique for planting lilacs is different from planting other bare-rooted plants, and the roots develop above the graft union to form a strong tree.

Best conditions

Lilacs need a cold, frosty winter to bloom to perfection. They thrive in slightly alkaline soil and detest acidic conditions. A dressing of lime every autumn where soil has a pH of 6.5 or lower stimulates heavy, leafy growth.

I remember seeing some stunning lilacs in North America, with rich green foliage and masses of huge blooms. The panicles were so large and heavy that they dragged the branches down almost to the ground. The secret was in the soil. It had a pH of 8, which is surprisingly alkaline, but they clearly loved growing in such conditions.

Lilacs develop lovely deep roots over time, and mature specimens are remarkably drought tolerant. However, they will certainly benefit from a slow monthly soak during dry summers.

PHOTOS IAN HOFSTETTER/PETER CUNDALL

There are more than 1500 named lilac cultivars, some quite vigorous. The most commonly grown in Australian gardens are cultivated varieties of Syringa vulgaris, many of which have been developed in France. Most of these have rich green, heart-shaped leaves and, even when not in flower in late spring and early summer, make outstanding garden specimens. They are excellent hedging plants which, when planted 3m apart in a row, grow to form a superb privacy screen.

Dig an extra-deep Place a stake
hole. Form a mound across the hole to
at the base of the hole check that the graft
and sit the roots over it. union is below ground.

Backfill the planting
hole with soil, making
sure the graft union is
buried. Water well.

Perhaps the most amazing thing about lilacs is their longevity. I’ve seen some plants that were well over 100 years old, and they were still flourishing.

Grafted plants

Lilac plants are best purchased in winter, while dormant, and are usually sold as bare-rooted plants. Although the trend overseas is to grow them on their own roots, most lilacs sold in Australia are grafted onto common lilac or a fellow member of the Oleaceae family, such as privet. This means that if they are not planted correctly, suckering can be a constant problem.

When plants are grafted onto common lilac stock, the leaves of suckers look similar to the main plant, making it difficult to identify them as suckers. If not controlled, suckers invariably overwhelm and swamp less vigorous

did you know?

+ Purple lilac flowers are thought to symbolise the emotion of being in love, while white lilacs represent youthful innocence.

cultivars, and gardeners often don’t realise this is happening until inferior flowers of the wrong colour appear.

However, when privet is used as rootstock, the suckers are easily identified and they can be cut back hard. This pruning, which needs to be carried out when suckers are actively growing in summer, sharply reduces their vigour, giving good control.

Plants are grafted onto privet stock because of a well-understood condition called delayed incompatibility. Most privet grafts have a relatively short life of five to 10 years – just long enough for the named lilac to form its own roots before the rootstock and graft fail. Known as ‘nurse-grafting’, this technique is used with certain plants that are hard to strike from cuttings.

It is possible to prevent suckering by planting lilacs in a manner quite different to that used for most other bare-rooted trees or shrubs. For this method to succeed, the lilac plants need to be planted in well-drained ground that doesn’t contain any subsoil clay. Dig an extra-deep hole and bury the plants deeper than you normally would, so the graft union is buried below the surface. Although this is contrary to the normal rules of good planting, it actually works.