Some plants, and these plants aren’t all bad, have a characteristic about them that prevents other plants from growing. They exude certain chemicals that prevent seeds from germinating or decimate seedlings and saplings. The adjective used to describe these plants is allelopathic.

Allelopathic plants can be problematic at times. The chemical excretions from the roots of certain trees, for example, will prevent other plants and trees from growing well for years, even after they’ve been cut down. However, these qualities are also being studied as natural weed suppressors to replace those nasty weed killers like Roundup.

Whether we view these botanical qualities as good or bad, as growers of plants, we must be aware of which plants are allelopathic. Then, we have the opportunity to either avoid them or use them strategically. A little knowledge gives us some power to act sensibly in the garden.

Source: EdibleAcres/Youtube

Trees

There are quite a few common, some quite useful, allelopathic trees. This list is by no means exhaustive, but it points out some of the allelopathic trees to be aware of.

  • Black Walnut is infamous for having a compound—juglone—that suffocates plants trying to grow around it, but of course, walnut trees provide healthy food for us.
  • Black locust is a fantastic timber tree, particularly for things like fence posts, because the wood is naturally rot-resistant. It is allelopathic.
  • Tree of Heaven is an invasive species introduced to the US, both coasts, as a fast-growing, ornamental shade tree. Unfortunately, it is known to be allelopathic and spreads aggressively via far-reaching runners.
  • Other trees that can be put on this list include sugar maple, rhododendron, sumac, and elderberries.

Grasses

For those trying to build gardens on former lawns or mulching with grass clippings, it’s worth knowing that popular select grasses can be allelopathic as well. Tall fescue, Bermudagrass, and Kentucky bluegrass are species commonly chosen for lawns, and all of them have allelopathic properties.

For lawns, this might be great because the point is to have only grass. However, using fresh/green clippings of these grasses might not be wise around seedlings. Mulch is supposed to suppress weeds from growing, but we don’t want it to release chemicals that endanger our vegetable plants.

Source: Daisy Creek Farms with Jag Singh/Youtube

Flowers

Some of our favorite flowering plants can have allelopathy as well. This list includes plants that are often grown in or around gardens for their pest-repelling qualities.

  • Forsythia is a favorite plant for those in search of early spring blooms. They put out loads of yellow flowers. In addition to being attractive and good for pollinators, the flowers and young leaves are edible. However, forsythia is allelopathic.
  • French marigolds are a common plant for flowerbeds and garden beds. It is often used and grown as a companion plant in veggie gardens because of its aroma and certain biochemicals it exudes to repel certain pests. It also has allelopathic compounds.
  • Sunflowers are another favorite flower for folks to grow. There are all sorts of choices, from the towering giant sunflowers to Jerusalem artichokes. These are also used for garden pest control as well, more as a distraction than a repellent.

Grains

Lots of the grains we eat and grow have allelopathic qualities. Some of these are grown as cover crops over the winter, so it’s worth knowing what’s what.

  • Barley, buckwheat, oats, sorghum, and rye are all commonly grown as cover crops in addition to the food they provide. While growing, these plants are allelopathic.
  • Wheat and rice are our two most widely consumed grains, and they, too, are allelopathic. This can be very important when using straw mulches.

Source: RED Gardens/Youtube

Brassicas

The brassica family is far-reaching and includes quite a few noted allelopathic plants. These plants may stunt the growth of other crops if they are planted together, so it’s important to recognize this when choosing what to grow together. Rapeseed, mustard, and radishes are often used cover crops, and all have known allelopathic qualities. Cabbage and kale can have some factors as well, and of course, the large outer leaves of these plants often drop into the soil to decompose.

Now that we have an extensive list, it’s worth saying that different types of allelopathy are not equal. Black walnut is a serious problem that will last for years. That said, some plants can live harmoniously unaffected—peach, plum, redbud, dogwoods, cherry—by the juglone. Others don’t stand a chance.

On the other hand, rapeseed’s allelopathic qualities dissipate as it decomposes, and in fact, the plant also releases growth-enhancing hormones. Sunflowers can be composted and then used, but leaving sunflower residue in the garden may become problematic because it takes a while to decompose.

The point is to be aware that allelopathy exists as it might be the cause of a persistent problem in the garden.

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