The Peril of Perennial Ageratum

One of the most memorable of the original ‘Star Trek’ episodes involves an itinerant space peddler who barters for an alcoholic beverage with a small, fluffy creature called a Tribble, the likes of which no one had ever seen before. The Tribble likes to be stroked; it coos and makes people feel good; perhaps too good. The Tribble’s chief downside is that is fecund to a fault. In the words, of Dr. McCoy, “Jim, as near as I can tell, these things are born pregnant.” Tribbles soon overrun the space station and threaten a shipment of a valuable grain seed stock. I won’t spoil the plot, but at the end of the episode the Tribbles are dispatched to a Klingon war ship.

Perennial ageratum

I was thinking about that episode yesterday as I spent the better part of an hour getting rid of my own Tribbles; specifically, pulling out patches of Conoclinium coelestinum, better known as perennial ageratum, from my garden. Thus far I have filled two, 50-gallon cloth barrels with the nasty stuff. I suspect my work is not yet done.

'Good' Ageratum houstonianum

A quick tutorial: Ageratum houstonianum, as anyone who grows summer annuals knows, is a wonderful filler plant for containers and a great ‘boots and socks’ border for beds. It has a lovely, bluish-purple flower than draws the eye. Alas, it dies with the first frost. It is a wonderfully well-behaved annual, prized for that unusual color. I can recommend it unconditionally.

Note the area circled in blue:
the perennial ageratum was
half the height of the Fothergilla
'Blue Shadow'

Now, let’s talk about perennial ageratum. While both it and its annual namesake are members of the aster family, their only common attribute is their similar-colored flower. Conoclinium is anything but well-behaved. It spreads by rhizomes; it spreads by flower seeds. It may even spread by word of mouth. But, once it is in your garden, controlling it is a full-time job come the end of summer.

To the best of my knowledge, neither Betty nor I have ever purchased a pot of perennial ageratum, yet this August it is everywhere in the garden. It is crowding out our beautiful Astilbe, upstaging our Fothergilla, and colonizing a bed of spring bulbs that, without human intervention, would be suffocated by a mat of roots. How did it get here? Maybe an itinerant peddler traded a sprig if it to one of our neighbors for an alcoholic beverage.

This false strawberry ground cover was
engulfed by perennial ageratum. Time 
will tell if I got it all.

What I do know is it has to go. But the Conoclinium has its own plan. Pulling it out is a piece of cake: it appears to come out with its roots intact. But, if you pull out a stand and then sift the soil, you’ll find pieces of roots and rhizomes cleverly left behind. I found that out because I thought I had completely eradicated perennial ageratum from the top of our driveway bed. Three weeks later, there were hundreds of replacement stalk coming into flower. (The plant apparently thrives in rain, of which we have more than our share.)  Now, whenever I remove a patch of Conoclinium, I immediately also dig around in the surrounding soil to see what may have been left behind. I am seldom disappointed.

Note the perennial ageratum
(circled in blue) hiding in
the Rudbeckia goldstrum

What is worse, the more I look, the more of it I find. The plant loves the sun. It also thrives in deep shade. It had insinuated itself in a bed of Potentilla indica (false strawberry) where it was keeping a low profile, as well as in a clump of just-past-bloom Clethra alnifolia (summersweet), where it had stretched to a height of four feet. Perennial ageratum is an ambitious interloper.

As of yesterday, I believe I have, at least for the moment, gotten it under control.  Which is, of course, a foolish statement. We’ve had two inches of rain in the past 24 hours. I know darned well the perennial ageratum is using the time to take stock of where its remaining troops are recovering. Reinforcements will be called in, perhaps from neighboring properties. I don’t know exactly when this war was declared. What’s more unsettling is that I don’t know if I’m winning or losing.