Surprisingly Invasive Herbs – Don’t Let These 6 Delicious Botanicals Take Over Your Yard or Ruin Your Garden

Invasive plants are problematic, non-native species that can cause harm in several ways. They tend to spread aggressively, which can be an issue on a large scale and also in the home garden.

Gardeners might be surprised to learn that some common herbs are actually invasive species. If you grow or plan to grow these herbs, consider the risks and either choose other herbs or take steps to minimize their spread.

Let's dig in to explore what herbs are invasive and what to do so they don't take over.

The Problem of Invasive Plants

To be designated as invasive, a plant must have two important characteristics:

1. Invasive plants are not native to the region in which they are invasive. They were introduced to the area, either on purpose or unintentionally.
2. Invasive plants cause economic or environmental harm, or even harm to human health.

An invasive plant often causes harm by outcompeting other plant species. They tend to grow more vigorously and spread more widely, crowding out either native plants or agricultural plants. There are several characteristics an invasive species might have that make it so successful:

• Produce and disperse a lot of seeds
• Long, spreading, and aggressive root systems
• Grow readily from disturbed soil
• Produce chemicals that inhibit the growth of other plants

Invasive plants can be a big economic issue in agriculture. Elsewhere, they cause environmental harm by overrunning and creating areas with fewer native species. In the home garden, invasive plants can take over your beds. They can also spread out of your garden and into nearby natural areas, causing harm.

What Herbs Are Invasive?

Among the many invasive plants that can be an issue in the home garden are several invasive herbs.

1. Mint

Mint is one of the most aggressive spreaders you’ll find in a garden, not just in an herb garden. Although it’s a delightful herb to grow with a nice fragrance and culinary uses, it will absolutely take over beds and your entire yard if you let it.

Mint spreads by both its persistent underground roots and its above-ground runners. Anywhere a runner touches the soil, it will put down literal roots and keep spreading. Even a small amount of root left in the soil can grow into a big, new plant that keeps spreading. Only grow mint in containers.

2. Lemon Balm

Another pleasant herb that you might not want in your beds is lemon balm. It smells delicious, and pollinators love the flowers, but it will crop up all over your garden if you’re not careful. It spreads by seeds that disperse readily.

You can avoid the invasive issue by growing lemon balm indoors. It thrives in sunny windows. If growing outdoors, remove the flowers before they have a chance to develop seeds.

3. Oregano

Oregano is popular for its many uses in the kitchen, especially in Italian dishes and on pizza. Unfortunately, it can grow quite aggressively and cause a problem in garden beds. This is a tough plant that can spread in low mats during good weather and in rich soil, but also during dry, even drought conditions.

Oregano is spread by underground rhizomes and seeds. You can grow oregano plants indoors or in containers, but take care to remove flowers before they can set seeds. If you grow it in beds, be prepared to aggressively cut back plants several times during the growing season.

4. Comfrey

Comfrey is a traditional medicinal herb with pretty purple flowers that attract pollinators to the garden. Unfortunately, it’s also an aggressive spreader that can be very difficult to eradicate once it takes root. It spreads by roots, and even a small piece left in the soil can become a new plant.

Comfrey also spreads by seed, so those pretty flowers will turn on you if you leave them in place. You can grow comfrey in a container, but also be sure to remove flowers before seeds develop. We like the garden snips from Modern Sprout, available on Amazon, for deadheading flowers and herbs.

5. Pennyroyal

Pennyroyal is an herb with traditional medicinal uses. It also has a strong smell that can be used to repel pests. Pennyroyal is easy to grow, which can make it tempting to use in any wet areas where other plants struggle, but be prepared to face an invasion.

Pennyroyal grows well in wet areas and spreads by runners, like mint. It creeps readily over soggy areas, like stream and pond banks and marshy areas, creating low, dense mats. If you have pennyroyal, you’ll need to spend some time every year pulling it out of areas where you don’t want it. A Fiskars ergo cultivator, available from Amazon, is great for pulling dense mats of roots and weeds out of the ground.

6. Fennel

Fennel is both a tasty vegetable and herb, but it can also be invasive. It spreads very readily from seeds, especially in mild climates and in soils that have been disturbed. This has made it a real issue in some areas, including California. It also cross-pollinates with dill and results in strangely flavored plants.

If you don’t live in an area where fennel has become invasive, you can probably grow it safely, but take care. If you let the seeds disperse, it might just take root in and then overtake your garden. Deadhead the flowers before they have a chance to develop seeds.

How to Grow Herbs Without Them Taking Over

Luckily, you don't have to completely write off these invasive herbs. Control invasive herbs by deadheading and cutting them back before they set seed or spread. You can contain them to reduce their aggressive impact on the rest of your garden.

Container Gardening

Container gardening for invasive herbs can be done one of two ways. You can plant individual herbs into individual containers and leave them above ground, or you can recess the containers into the ground. If you decide to recess your containers, it's best to use simple undecorated containers made of plastic, such as the ones you purchase plants and seedlings in. Don't recess the invasive herbs in the same container that you purchased them in though. Use a container that is a size or two larger to ensure that your plants have room to grow and mature.

To recess a container for invasive herbs, dig a hole large enough for the entire pot to fit into, leaving the lip (top portion) of the container sticking out approximately 1 or 2 inches (2.5-5 cm.). Be sure that your container has drainage holes. Fill the bottom of the container with gravel or Styrofoam pellets to allow proper drainage of the pot. Add potting soil and then plant your herb into the buried container. Your container-gardened herbs will need to be dug up every year or two and divided to keep them from becoming root bound.

Compartment Gardening

Compartment gardening can be done by placing borders around your invasive herbs that are planted directly into the garden. You can create separate compartments for your invasive herbs using metal or plastic edging around them. The edging must be buried fairly deeply, to keep your herbs from spreading.

Container Herb Garden Essentials

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