You Should NEVER Grow These Plants in Your Garden

If you think you can grow any plant variety in your garden, think again! You may come back from a nursery with seedlings smiling and ready to plant them, not knowing that you are bringing an “enemy” into your green haven! Not all annuals, perennials and vegetables are friends… While the newcomer will grow healthy and strong, your lettuce or petunias or tulips will start to suffer. What’s happened? You have chosen an allelopathic species! And the only choice you have left is to try to eradicate it, but it may be too late, or, in the best-case scenario, you will have lost the season…

Beware of allelopathic plants! They can spell disaster for your decorative and vegetable garden, but how do they work? And which are these varieties, and how can you deal with them? Let’s find out!

What’s an Allelopathic Plant?

Whatโ€™s an Allelopathic Plant

It looks like a big word, but allelopathy is a common phenomenon with plants; basically, these are varieties that produce allelochemicals, chemical substances that prevent or slow down the growth of other plants, or sometimes they stop them from flowering and fruiting.

They spread these chemicals in the soil, stopping or hampering the growth of their green neighbors… It’s a matter of competition. So, what can you do?

What to Do if You Have an Allelopathic Plant in Your Garden

What to Do if You Have an Allelopathic Plant in Your Garden

Unfortunately, you may already have some allelopathic plants in your garden, because some are very popular. For example? Azaleas and rhododendrons (Rhododendron spp.), English laurel (Prunus laurocerasus), elderberry (Sambucus spp.), sumac (Rhus spp.) and forsythia (Forsythia spp.), or fennel (Foeniculum vulgare) in your backyard!

It’s hard to do without them, if you are growing them already, and you can, of course, but keep them at a distance from other plants, trying to isolate them, and maybe think about having them in containers.

So, let’s see some of these competitive plants, and how to deal with them.

1: Fennel (Foeniculum vulgare)

Fennel (Foeniculum vulgare)

Fennel is one of the best-known allelopathic vegetables, and if you plant it with your tomatoes or peppers, your crop will be really poor. However, it is so delicious that it is hard to do without it. What can you do?

Never grow fennel in the same bed as other vegetables, and if you have a sloping garden, plant it at the bottom (your neighbor will never know why your harvests are much better than his or her!) This way, the water will take the allelochemicals away from your land.

Fennel is also a small vegetable, so you can grow it in containers (not raised beds), making sure there is a tray or saucer under them, and the water does not disperse the chemicals.

If you have an isolated bed, then that’s a good place for fennel.

2: Sunflower (Helianthus annuus)

Sunflower (Helianthus annuus)

Surprised? We all like to sow some sunflower seeds here and there in our garden, but these lovely flowering giants are really powerful suppressors of other varieties, in fact, you will hardly see a blade of grass growing in sunflower seeds.

If you want sunflowers in your garden, grow them in an isolated group, maybeat the back, where they can give you a brightly colored backdrop, and at least 2 feet away from your precious perennials, annuals or vegetables.

3: Mustard Greens (Brassica juncea)

Mustard Greens (Brassica juncea)

Mustard greens are so allelopathic that they are used as cover crops to suppress seeds! So, if you enjoy them in your meals, do not grow them next to any other vegetable variety and never use them as companion plants.

Most brassicas are lightly allelopathic, but not enough to compete seriously with other strong vegetables, but mustard greens are strong; keep them isolated.

4: Radishes (Raphanus sativus)

Radishes (Raphanus sativus)

Now you will think I am kidding you; you have never noticed that radishes stop any other vegetable’s growth. And it is true, they are not allelopathic while they are growing, in fact. But they are once they are decaying!

Here, the solution is simple; always uproot all your radishes at the end of the harvest or when they start dying; as soon as their leaves start turning yellow, remove them.

5: English Laurel (Prunus laurocerasus)

English Laurel (Prunus laurocerasus)

English laurel is actually toxic to humans if you do not cook it (it’s hallucinogenic) and it isn’t any kinder to its green neighbors. If you have a shrub (also trained into a tree) you will have noticed that almost nothing grows underneath it, because it is strongly allelopathic.

So, if you have an English laurel in your garden, you will have to give it its space; do not grow anything under its crown, and at least 3 feet (1.0 meter) away from it. By all means, keep delicate plants as far as possible.

I wouldn’t advise growing a laurel in a small garden, and even in medium sized plots, no more than one individual shrub, and in a corner.

6: Mint (Mentha spp.)

Mint (Mentha spp.)

Mint is quite a dodgy herb to grow in your garden; if you have tried it already, you will have seen that it tends to spread and take over. And it does it not just because it is very vigorous, but because it is really strongly allelopathic.

But how can you renounce its amazing aroma and flavor? Grow mint in containers, especially if you have a small garden; this is the best solution.

7: Forsythia (Forsythia spp.)

Forsythia (Forsythia spp.)

Forsythia is such a spectacle in early spring! Unfortunately, neighboring plants may think differently, and in fact, you won’t be able to grow tulips or lilies near it. It is allelopathic, of course.

If you grow forsythia, do not grow annuals or perennials near it; do like you see in professional gardens – where do you meet this golden flowering shrub? As a hedge, or in isolation at the end of a lawn!

8: Cucumber (Cucumis sativus)

Cucumber (Cucumis sativus)

Now, I am here to debunk a myth. If you already knew about allelopathic plants, you may think that cucumber is one of them, and known for it. And it is true, but it is not a very strong one! And this is good news, because you don’t need to relegate it to that corner at the back of your vegetable garden.

However, do not grow weak vegetables under or near it. Garlic, onions, cabbages etc. are all fine, but no lettuce or spinach will do well near or under cucumbers.

9: Perennial Rye (Lolium perenne)

Perennial Rye (Lolium perenne)

Perennial rye? What is it? It’s a type of grass, often used for lawns… In fact, it is popular because on these green carpets you will never see other little plants growing. Which is good news, if you don’t like variety.

It is also true that you cannot get wildflowers, violets, or weak bulbs growing on it… So, weigh the pros and cons.

10: Elderberry (Sanbucus spp.)

Elderberry (Sanbucus spp.)

Elderberry is a strong and vigorous shrub, with an amazing fruiting ability, but it doesn’t get on well with neighbors – it too is strongly allelopathic, and you will notice that virtually nothing grows under it, and little nearby.

Keep elderberries away from weak and allelopathic sensitive plants, and we will see which ones these are later on in this article.

11: Azalea and Rhododendron (Rhododendron spp.)

Azalea and Rhododendron (Rhododendron spp.)

Azaleas and rhododendrons will stop any smaller plants growing under them, annuals, perennials and smaller shrubs. So, keep this in mind if you want them in your garden, and maybe choose a small variety if you love them but you do not have space, and give them some room.

On the other hand, azaleas and rhododendrons do not compete with trees, so you can easily grow them under them!

12: Allelopathic Trees

Allelopathic Trees

Finally, before you go out and buy a tree to grow in your garden, beware of these, because nothing or very little will grow under them!

  • American sycamore (Platanus occidentalis)
  • Black cherry (Prunus serotina) can inhibit herbaceous plants but not stronger ones.
  • Black walnut (Juglans nigra) inhibits many plants, especially tomatoes, peppers, and potatoes.
  • Eucalyptus (Eucalyptus spp.) is a really powerful allelopathic tree – nothing grows underneath it.
  • Oak trees (Quercus spp.) have leaves that produce tannins when they fall, inhibiting lots of smaller plants.
  • Pine trees (Pinus spp.); few plants can grow under them because the needles acidify the soil.
  • Red maple (Acer rubrum); when its leaves decompose, they produce allelopathic chemicals that stop the growth of herbaceous plants.
  • Southern magnolia (Magnolia grandiflora); both the leaves and the roots produce allelopathic chemicals that stop most plants from growing under it.
  • Sugar maple (Acer saccharum) stops the growth of herbaceous plants.
  • Tree of heaven (Ailanthus altissima) can stop the growth of under plants not just underneath its crown, but nearby as well.

So, now we have seen the most important allelopathic plants and how to behave with them in our front and back gardens, let’s see the most “vulnerable victims”, as I promised…

Allelopathic-Sensitive Crops and Decorative Plants

Allelopathic-Sensitive Crops and Decorative Plants

Not all plants (crops or decorative) are as vulnerable to allelopathic species; some are stronger, and some are weaker. You need to know this because you don’t want the most vulnerable varieties anywhere near these “unfriendly neighbors” in your garden.

Lots of the tender looking plants are really susceptible to allelochemicals, for example, starting with crops:

  • Lettuce, all varieties; they will strive to survive altogether.
  • Peas, they look tender, they are tender, they are vulnerable.
  • Tomatoes, they look strong to us, but they will really suffer next to allelopathic plants; their growth will be stunted, and the crop will be really bad.
  • Eggplant, same as with tomatoes.
  • Peppers, same story.
  • Beans will have a really stunted growth if grown near an allelopathic variety.
  • Carrots, allelochemicals stop root growth, so you won’t get much of a harvest.
  • Asparagus; its shoots are really tender and they will fail or disappoint you.
  • Alfalfa’s germination is blocked by allelochemicals – no point in sowing it altogether.
  • Potatoes, being root vegetables, will suffer a lot.
  • Brassicas, like cabbage cauliflower etc., although they are very mildly allelopathic themselves, they will have their growth reduced, and they will give you a bad harvest.     

Now, thinking about your decorative garden…

  • Hosta, with its tender leaves, is a bit like lettuce…
  • Astilbe will survive, but with poor leaves and blooms.
  • Daylilies too will survive, but they will struggle growing and blossoming.
  • Dahlias will hardly blossom, and the tubers will not grow well.
  • Most bulbous perennials, like lilies, tulips, hyacinths, crocuses etc., are better off as far as possible from allelopathic plants; their bulbs will shrink and their flowers will be few and far between.
  • Most annuals will suffer very much if you grow them under or near allelopathic varieties.
  • Hydrangea, yes, this strong and vigorous shrub will grow little and blossom even less with the wrong neighbors.
  • Peonies are very vulnerable to allelochemicals; you won’t get many blooms out of them, if any at all!
  • Lilacs too look very strong and vigorous, but not near these competitors, especially if big.

So, as you can see, growing allelopathic plants in your front garden reduces your choice of decorative varieties quite a lot!

On the other hand, and on a final note…

Get Allelopathic Plants to Sort It Out Between Them!

You may now wonder, “What happens if I put allelopathic plants together?” That usually works, because they are strong enough to “fight” underground, especially if they are not the most aggressive.

You can grow a pine next to a eucalyptus; both will grow well, but nothing will grow under them.

Thinking about your vegetable garden, for example, alliums (garlic, onions, shallots etc.) and brassicas (cabbage, Brussels sprouts, cauliflower etc.) are all mildly allelopathic, and you can easily grow them together as companion plants, and they will be perfectly fine! They even have complementary shapes!

Your Garden Is Like a Kindergarten!

So, you thought your plants were all getting on and for some strange reason some were not doing well? Now you know that under your feet, they were bickering! You need to treat them like kids; divide the ones that don’t go well together, and have a peaceful classroom…

Amber Noyes

Written By

Amber Noyes

Amber Noyes was born and raised in a suburban California town, San Mateo. She holds a master’s degree in horticulture from the University of California as well as a BS in Biology from the University of San Francisco. With experience working on an organic farm, water conservation research, farmers’ markets, and plant nursery, she understands what makes plants thrive and how we can better understand the connection between microclimate and plant health. When sheโ€™s not on the land, Amber loves informing people of new ideas/things related to gardening, especially organic gardening, houseplants, and growing plants in a small space.

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