lilac (Syringa vulgaris) photo by Schoeband

My 3-year-old lilac tree is doing so well and almost in full bloom. It’s still low and very bushy. My question is: When it’s done blooming, can I prune it? How do I do this? Also, please tell me a feeding schedule and what food you recommend.
– Rhoda Rose
Woodland Hills
You must be growing a lilac cultivar such as ‘California Rose,’ which does not need a cold winter, unlike traditional lilac types, to bloom. Typically, lilacs start to bloom at the end of February or in March and have traditionally been used as an indicator plant for the end of freezing weather.
‘California Rose’ is one of a number of lilac cultivars that were hybridized from the lilacs growing at Descanso Gardens in La Canada Flintridge. Unlike most lilac (Syringa) cultivars, Descanso hybrids do not require much winter chill in order to bloom in the late winter or early spring.
As an early bloomer, lilacs are best pruned when flowering ends. Cut back each flowering shoot to a healthy pair of opposite leaves. Shoots that did not flower should not be pruned other than to shape the plant.
As for fertilization, it is really an afterthought where lilacs are concerned. Too much nitrogen leads to excessive, leafy vegetative growth at the expense of flower production. If you do fertilize, which you should not do more than once a year, make sure the product contains twice as much phosphorus, which stimulates flowering, as nitrogen.
On any fertilizer product, you will see three numbers separated by dashes. The first number is the percentage of nitrogen in the bag, the second number is phosphorus and the third potassium. Make sure the second number is at least twice as big as the first number.
‘California Rose’ is a widely grown cultivar that, if not in stock, should be available by special order from most nurseries in our area.
If you visit Descanso, make sure you take a look at the lilac collection there. People associate the Descanso Gardens with a rapturous camellia forest growing under native oaks, but the lilacs, easily missed because they are off the beaten track of the casual visitor, should not be overlooked. To appreciate the Descanso lilacs in full bloom, plan a visit for March or April.
Lilacs are known for their longevity, living 100 years or more in their habitat of southeastern Europe, on rocky Balkan slopes in Romania, Bulgaria, Serbia and Macedonia.
A primary reason for their staying power is their strong suckering tendency. New growth is constantly pushing up from the soil, which also makes propagation easy. You can create new plants simply by digging up young suckers along with their roots and transplanting them to other parts of the garden.
Before conducting this procedure, make sure that the lilac you want to propagate is growing on its own roots. If you have a grafted plant, the suckers will develop into lilacs whose characteristics may disappoint you. In such cases, suckers should be regularly dug up and discarded instead of propagated.
Lilacs may be grown into magnificent hedges 5 to 15 feet tall, depending on variety. Few people, however, take this horticultural plunge, since lilacs are deciduous and do not bloom for more than a few weeks in the spring.
In addition, they can burn in our hot summers and should be given good ambient light without too much direct exposure to southern or western sun. Too much shade, however, will result in mildewy leaves.
Lilacs require fast-draining soil to grow well and often live for only a few years due to inadequate soil drainage. However, they are not drought tolerant and require steady soil moisture.
We have a very old wonderful catalpa in our front yard. We have had it trimmed at times and have had different opinions as to when it is best to trim a catalpa. Would you have any thoughts on this? Trim after the leaves are off or while the tree is full, green and beautiful?
– Kate Prater
Encino
The tree to which you refer (Catalpa bignonioides) has fragrant, tubular flowers shaped like slightly bent trumpets. Flowers are white with maroon and yellow markings inside. The shape of catalpa flowers will remind you of those seen on jacaranda trees (Jacaranda acutifolia) and on trumpet vines (Clytostoma, Distictis, Tecoma and Bignonia genera), which are all relatives of catalpa.
Catalpa also has unusually large, heart-shaped leaves and long, brown, skinny seed pods. Seeds germinate easily. The tree grows to a moderate size of 30 to 40 feet and is more functional as an ornamental, as opposed to a shade, tree.
Catalpa is a highly adaptable tree, growing well in all soil types. It may be completely defoliated by a caterpillar (moth larva) that makes excellent fish bait. In fact, it is grown in some locales for the express purpose of bountiful fish bait.
Following defoliation by this pleasantly pesty caterpillar, catalpas grow a complete set of new leaves, and may do so several times during a single growing season.
As to your question, the answer is to prune “while the tree is full, green and beautiful.”
Ornamental shrubs and trees that bloom in the spring or early summer, especially deciduous types such as catalpa, are pruned soon after they flower. The reason for this is that next year’s flower buds are produced on shoots that grow immediately after this year’s flowers fade. This means that if you wait to prune until after leaves fall, for example, you will be cutting off next year’s flower buds, all of which are already on the tree, resulting in a less spectacular bloom.
There is a notable exception to the law of pruning spring flowering trees after bloom and that is fruit trees. If you prune a fruit tree such as an apple, plum, peach or cherry after it blooms, you will reduce the quantity of the current year’s harvest since fruits develop from pollinated, faded flowers.
As for summer flowering shrubs and trees, such as crape myrtle, they typically bloom on current season’s growth, so winter pruning at the end of winter dormancy is advised.
Tip of the week
Lilac flowers lose their freshness after only three to five days, so extra care is required to maintain them in a vase. Maximize the longevity of any cut flower by changing the water daily and adding a floral preservative. You can make your own flower-preserving formula by mixing 1 teaspoon bleach, 1 teaspoon sugar and 2 teaspoons lemon juice into 1 quart of lukewarm water. As soon as cut flowers are removed from the plant, they should be placed in the prepared water. Ideally, an inch or two is cut off the bottom of the floral stems, underwater, at a 45-degree angle. Before placing any stem in the water, make sure that no leaves will remain underwater as this will foul the water and bring rapid deterioration of the leaves and flowers in the arrangement.