How do we fix our flower beds, meadow and borders after Goldenrod has invaded?
Question:
Goldenrod (canadensis) has invaded flower beds, meadow, and woodland border. At first I didn’t mind, but the goldenrod is turning into a monoculture. I have read that part of its success is because it is allelopathic. What is the chemical(s) and is there a study or list of plants that are not affected? For example, there are such lists for black walnut/juglone. I know and can see that a few asters are okay with the goldenrod, but that seems to be it. Most everything, even established ferns and sweet fern, are quickly taken over.
Answer:
First, let’s look at black walnut. The allelopathic effects of the species have been a widespread belief among gardeners for a very long time, but the experimental testing of juglone has given inconclusive data. In laboratory settings, extracts of black walnut have absolutely demonstrated allelopathic effects on germination of seeds from other species. Field trials have alternatively shown very little real world impact of the black walnuts on nearby species. The reason for this disparity isn’t fully understood, but may have something to do with juglone 1) not being found in living walnut tissue, forming instead from decomposing precursor compounds in dead walnut tissue, and 2) not permeating soil very well (though it does spread pretty well through potting mix, which has a higher permeability). Washington State Univ Extension has a nice summary of the research on black walnut and juglone here.
Pivoting to goldenrod, I’m sorry to say I’ve only had time to run a quick search, but I did turn up one study here that seems to give similar results to those of juglone–namely, definite allelopathic effects in lab settings from extract (though varying by species), poor data to demonstrate a real effect in field trials. Regardless of the situation, goldenrod’s ability to spread in the landscape is more likely to be its easy tolerance of a wide variety of conditions, its high seed count and germination rate, and its propensity to reproduce vegetatively once established. I don’t believe they named the exact chemical responsible for the allelopathy.
Neither of these is intended to discount any experience you may have had in your garden with either of these species, as gardens and ecosystems are complex entities. It’s just a summary of what has and hasn’t been successfully reproduced through experiment.
To practical matters, if the goldenrod is giving you trouble taking over, this blog entry from the Cornell Univ Extension is a nice, short treatment of the options for goldenrod. They can resprout from roots, so hand removal is really only effective for individual plants, with care taken to extract as much of the root mass as possible. For larger patches, repeated mowing over several seasons can get the goldenrod under control, always taking care to prevent them from flowering and going to seed. If it’s taking over, you definitely don’t want any new seeds going into your soil seed bank to germinate new plants. If you think you need to use a chemical intervention, a late summer/early fall (when perennial plants are pulling above ground resources down into their crown and roots to prepare for winter) application can be effective (see the link above). As always with pesticides, read carefully and follow conscientiously all instructions on the label.
Happy gardening.