Wood Ash and Lilac Soil pH: Do Lilacs Like Acidic Soil?

'The Time Has Come', The Walrus Said, 'to Speak of Many Things—Of Wood Ash and Lilacs and How the Blooms to Bring…'

Q. I heard you say recently that Lilacs require acidic soil, much like azaleas. I was curious about your source of information and data on this subject. A lot of us spend a lot of time, money and effort trying to get our plants to grow and live long and successful lives. It is important that we receive good and accurate information from those who purport to be experts. We look forward to elaboration on this topic.

---Tom in Greenville North Carolina

A. Oof—I feel like I'm back in Grade School and the teacher just pulled me into the front of the room by my ear. Anyway, Tom asks a legitimate question. He's referring to a call on a previous show from a guy named Jack up in State College, PA about using ashes from a wood stove on a lilac. People who burn wood generate a lot of ash and they're always hoping they can use it on their garden and landscape.

…Despite my almost weekly warnings that "just because you have a lot of something doesn't mean it's good for your plants…" (Hey! That can be on our next Public Radio Pledge Drive mug! With a picture of somebody who just tilled a big pile of wood chips into their garden and a tomato plant with X's over its eyes!)

Anyway, like a lot of people, Jack had heard that wood ash was good for lilacs. And what I should have said was the much more temperate 'you have to be careful with wood ash—it's really alkaline, and most plants want acidic to neutral soil'. But if I only spoke temperately, my show would have closed 14 years ago. So instead I went out on the very long and spindly 'lilacs want acidic soil' limb.

And I was wrong.

When I got Tom's email, I went back and checked a big article we ran on lilacs in ORGANIC GARDENING magazine back when I was the editor. Everything in the magazine was seriously fact checked by our research staff back in those days. And the article specifies an ideal pH for lilacs of 7 to (gulp) 8…

Eight is alkaline. So are wood ashes actually good for lilacs?!

Yes. Maybe. Sometimes. Probably.

If you have acidic soil, a little bit of ash would almost certainly be beneficial. But if your soil is already alkaline, like in Oklahoma, parts of the West Coast and other areas with lesser amounts of rain, no.

I'd like to be able to say that I was at least 'half right' because of that exception, but the call was from Pennsylvania, where acid soil is the rule. And lilacs generally don't do very well in warmer climes to begin with, which may be why a guy in the Carolinas would be paying really close attention when the topic came up.

So if Tom does have struggling lilacs in NC, here's my 'I'm not making it up off the top of my head this time; I just read (and re-read) a fact-checked and very thorough lilac article' advice to him—and really to everyone who doesn't think their lilacs are blooming enough.

First, be patient; it's normal for lilacs to take four or five seasons in the ground before they really start to flower well. But it's a good investment; lilacs are very long-lived plants…

Then yes; if your lilac is well established and you don't think you're getting enough flowers, absolutely test the pH of the soil and add some wood ash--or lime--if it's low.

But you won't get really good flowers unless the plant also gets lots of sun. I have a lilac that took the typical three or four years in the ground to produce some flowers, and then produced enough every season to earn its keep. Then, beginning around year ten, it was covered in flowers every Spring; because, I have come to believe, it grew tall enough to escape the shade of other nearby plants and get full sun all around.

But most experts warn that a lilac in the South will generally never bloom as well as the same plant in the North. Most lilacs are rated as winter hardy down to the absolute coldest zones—USDA Growing Zones 3 and 4. And lilacs not only survive those harsh winter temps—they thrive on them. The more winter freezes the plant endures, the more likely it is to produce copious blooms.

But that doesn't mean folks down South shouldn't give it a try. Things like providing a little afternoon shade and extra watering should help to keep the basic plant alive during particularly sultry summers. And choosing the right variety up front might be able to help you also enjoy a good amount of blooms, as some varieties are said to have lower 'chilling requirements' and thus do better in zones 7 and higher. I'd ask my local Extension Service for specific variety recommendations; they're always your best source for that kind of truly local information.

And wherever you live, make sure to only prune lilacs immediately after the flowers fade in the Spring; lilacs set the following year's buds very early in the season. If mildew is an issue, remove entire branches to open up the plant and improve the airflow. Otherwise, just remove as many of the faded flower heads as you can reach; this 'deadheading' also leads to better blooms the following Spring.

As does—sigh—keeping the soil a little on the ALKALINE side. Thanks, Tom. I'll try and remember that from now on.

Bonus: Here's a nice little article about lilacs for the South from Tom's home state.