Gardening Poisonous Plants

Toxic Plants Don’t Make Other Plants Poisonous

By Larry Hodgson

Question: I have a big, beautiful brugmansia I lug outdoors for the summer. It grows in an appropriately large tub and I was thinking of planting nasturtiums in it. I wanted to grow them for their edible leaves and flowers.

However, I know brugmansias are poisonous. Will growing edible plants in a pot with a poisonous one make them toxic too?

Sylvie

Answer: No.

Poisonous plants are toxic because they contain toxic substances of their own making. This is a natural protection they evolved over the eons . . . usually to keep pests (like humans) from eating them! However, these substances are not transferred intact to the soil. In contact with dirt, they quickly break down into harmless molecules. Nor could any plant absorb the complex molecules that make up plant poisons even if they were any still present in the soil. Plant roots can only absorb the simplest elements.

Let’s take a specific look at brugmansia or angel’s trumpet (Brugmansia spp.). The main toxic compound in this plant, present in leaves, flowers and stems, is an alkaloid called scopolamine (hyoscine). There are others, but they’re similar alkaloids and react in the same way. The chemical formula of scopolamine is C17H21NO4. And it is indeed highly poisonous. But plant roots can’t absorb chemicals as complex as C17H21NO4. . .

Scopolamine quickly breaks down into perfectly safe elements once the leaf or flower is dead. Since it only contains carbon (C), hydrogen (H), nitrogen (N) and oxygen (O), it will turn mostly into carbon dioxide, water, nitrogen and oxygen. Those are the simple everyday elements that make up most plants . . . and human beings. Nothing to worry about!

You could even use brugmansia leaves as a mulch or a soil amendment and they would absolutely not make your nasturtium (Tropaeolum majus) leaves and flowers poisonous!

And there is no reason not to add dead brugmansia leaves and blooms to your compost bin either.

Lots of Other Examples

Apple trees with balls of mistletoes
Apple trees with poisonous mistletoes still produce edible apples. Photo: panasse, depositphotos

In the wild, close encounters between edible plants and poisonous ones are the norm. Plants simply don’t line up with the edibles on one side and the deadlies on the other. They all grow together. Imagine a poor squirrel trying to figure out if it could eat that acorn that grew too close to a holly or yew . . . 2 poisonous trees. It wouldn’t make sense!

And gardeners place poisonous plants next to edible ones all the time. Potatoes are poisonous in all their parts except their tubers. Yet, no one suggests removing them from the vegetable garden! Rhubarb is poisonous, yet a common vegetable garden plant. Some gardeners even claim stinging nettles make great companion plants for vegetables and plant them in their gardens. (Not me, though! I will knowingly not plant anything likely to sting me in my garden!)

Could a Poisonous Plant Toxify Neighboring Plants in Other Ways?

This is a long shot, but maybe pollen could fall from a brugmansia flower and land on a nasturtium leaf or flower. In that case, the part eaten might be just a little bit toxic.

However, is that even a problem? I mean, you could solve that simply by rinsing the flowers and leaves with water before you eat them. Something you undoubtedly do anyway!

Just Do It!

So, go right ahead and plant nasturtiums under your brugmansia. Just rinse the resulting leaves and flowers before you eat them.

Top photo: elenarostunova & alexusha2008, depositphotos

Garden writer and blogger, author of 65 gardening books, lecturer and communicator, the Laidback Gardener, Larry Hodgson, passed away in October 2022. Known for his great generosity, his thoroughness and his sense of humor, he reached several generations of amateur and professional gardeners over his 40-year career. Thanks to his son, Mathieu Hodgson, and a team of contributors, laidbackgardener.blog will continue its mission of demystifying gardening and making it more accessible to all.

11 comments on “Toxic Plants Don’t Make Other Plants Poisonous

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  5. Reminds me of the common concern I’ve read about composting rhubarb leaves. Humans eat the stems no problem, but the leaves are toxic for us. Then there are people who fret about composting the leaves since they’re concerned that they will “poison” the compost. Very odd idea, but I’ve heard it so many times.

  6. As long as you do not mix up the leaves, berries & stems, when harvesting.

  7. As mentioned some of our favourite veggie plants are super toxic yet we can eat some of their parts just fine. Tomatoes for example. Nature is clever.

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