Q. My lilacs are really overgrown, and only flower near the top. How do I get them shorter and flowering down where I can enjoy them more??
It sounds like it's been a while since you've pruned your lilacs, so you may need to take some drastic measures. Overgrown deciduous shrubs can be gradually shortened through rejuvenative pruning.
The best way to do this is to remove 1/3 of the older, woody stalks for the next 3 years. Start with the ones that are the most "objectionable", like the ones that lean outside the body of the plant, or the ones that grow much higher and stick out the top. Be brutal! Cut these stalks to within 6" of the ground. They may re-sprout, but this is OK, as it will give you young shoots later on to choose from.
You'll probably need to remove a few of the younger interior shoots, but you'll be leaving most of them for now to develop. You'll likely be pruning them out at a later date when they, too, become older, but since lilacs don't tend to flower on this younger growth, you have to let them mature enough so that they produce later on.
Once you've rejuvenated your lilacs over the next few years, prune them annually to keep the older stalks to a minimum, and continually "groom" younger stalks to take their place. A little pruning each year saves you a lot of time overall, and it's better for the plant than trying to catch up on several years of neglect.
Q. We just bought our first home, and there are some beautiful lilacs in our backyard that look like they've been pruned regularly. I read a book that shows how to prune them to keep them this way but I don't know when they should be pruned. What's the best time?
A. Good for you for doing some reading! With the availability of information at the library and on the Internet, there doesn't seem to be a topic you can't learn about.
Prune spring-flowering shrubs within 2-3 weeks of when they've finished blooming if you want to preserve the next year's flowers. This is because they flower on the previous year's growth, so if you wait too long they'll have grown and already set next year's flower buds, and you'll be pruning some of them off. This won't hurt the plant, but let's face it, we like to enjoy the flowers so let's not cut them off!
However, if you have overgrown shrubs it's best just to give them what I call the "Marine crew-cut". This means cutting them close to the ground before the spring growing season and allowing all brand-new growth to come up. I did this with my forsythia at home, and they grew 6-8' that year!
Other shrubs that will tolerate, and actually enjoy, this type of pruning are mockorange and privet. Again, this type of pruning I reserve for the really overgrown shrubs. Shrubs that have been pruned regularly and properly only need annual maintenance pruning, which consists of removing older stalks as they become too large, and allowing younger shoots to develop and take their place.