How often should I water and what should I feed the lilac bushes to make them flourish? Some have lichen on branches.

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Lilacs are the original "plant it and forget it" shrub. You'll sometimes find them in the US growing in the middle of nowhere, which marks where a farmhouse once stood 100+ years ago.

I've grown lilacs pretty much my entire life. They don't mind not being mulched and not being watered except by rain. The biggest issue with lilacs as they age is to ensure that they're pruned properly. To help your lilacs thrive, give them a 2=3" mulch of arborist wood chips every other year or so - do not use landscape fabric under the mulch. It is unnecessary and harms the soil. As the mulch naturally decays, it feeds the lilacs. The mulch also helps the soil retain moisture. As for watering, during a drought you can water them when you water the surrounding plants, but be sure to do it early in the day - late-day watering can lead to powdery mildew. Also, never remove suckers - you'll eventually need them.

If you have a lilac that grows suckers, you'll need to prune it when a number of the branches get to be about 2" thick. Follow the 1/3 over 3 years rule - cut 1/3 of the old branches to the ground each year for three years. The suckers you've let grow become the new main branches over the next few years. This type of pruning rejuvenates the shrub and enhances flowering. I usually do this in late spring, after the lilac has flowers. So-called "French" lilacs often do not sucker, so do not practice this pruning regimen on them.

As for the lichens - they're usually an indicator of good air quality and will not hurt the lilac or any other plant. There's no reason to remove them.

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