Competition for light, water, nutrients and space can drive plants to extreme measures. Since they don’t really have the option to act as animals do, like eating one another or running away, they must resort to more subtle techniques. Some plants grow fast, relying on speed to get ahead of the others. Some plants are shade tolerant, and will bide their time in the dark while the speedy plants slowly die off. Some plants are far more devious, these plants employ a strategy called allelopathy.

As defined by the Meriam-Webster dictionary – Allelopathy: (al·le·lop·a·thy) the suppression of growth of one plant species by another due to the release of toxic substances.

Left: Black Walnut - Juglans nigra | Right: Garlic Mustard - Alliaria petiolata

That’s right, some plants get ahead by killing the other plants! Yeah, it might not be too noticeable…but it’s effective. If you’ve planted a tomato near a walnut or have seen a forest understory invaded by garlic mustard, then you’ve witnessed its effectiveness.

Not all plants are allelopathic, and if they are, rarely does the chemical they release kill everything. Many plants find a way to live under the onslaught of deadly chemicals. For example, Walnuts (Especially Black WalnutJuglans nigra) release a chemical called Juglone, which inhibits growth of many plants including tomatoes, pines, and hydrangeas. However, most maples, junipers, wild gingers, and hostas seem to do fine.

Left: Spotted Knapweed - Centaurea maculosa | Right: Common Buckthorn - Rhamnus cathartica

Canada Wild Ginger (Asarum canadense) – has been observed to have allelopathic properties. It was even proposed by our Senior Horticulturalist – Michael Yanny – that it may inhibit the growth of buckthorn seedlings in woodland settings. In time with some examples, that fact may boost this plant’s value as a restoration and woodland gardening species.

Examples of allelopathic plants include:

So, whenever you are planting in a new garden or want to keep certain plants from coming in where they don’t belong, consider the power of ALLELOPATHY.