Who doesn’t enjoy fresh herbs, those aromatic plants that add such punch to our meals? Or treat our sniffles or upset stomaches? And they’re never fresher than when we grow them ourselves. That’s why herbs are presently so popular: everyone wants to try them. And most people find them easy to grow… at first. But many herbs have a major downside: they’re moderately to highly invasive and can quickly switch from being useful plants to becoming out-and-out garden thugs.
Two Categories of Weedy Herbs
There are two categories of potentially weedy herbs: those that produce creeping rhizomes or stolons (or sprout from broken pieces of root) that head off in all directions, soon producing offsets that surround and overwhelm neighboring plants, and those whose invasive habits are due to self-sowing, giving hordes of babies from the seeds they drop, hordes that can quickly threaten your entire herb garden.
Here is a list of the “main culprits” along with their preferred mode of invasion:
- Borage (Borago officinalis): seeds
- Caraway (Carum carvi): seeds
- Catnip (Nepeta cataria): seeds
- Chamomile (Matricaria recutita): seeds
- Chervil (Cerefolium anthriscus): seeds
- Chives (Allium schoenoprasum): seeds
- Comfrey (Symphytum officinale): seeds and root sections
- Coriander or cilantro (Coriandrum sativum): seeds
- Costmary (Tanacetum balsamita): seeds
- Dill (Anethum graveolens): seeds
- Fennel (Foeniculum vulgare): seeds
- Feverfew (Tanacetum parthenium): seeds
- Garlic chives (Allium tuberosum): seeds
- Horseradish (Armoracia rusticana): root sections
- Lemon balm (Melissa officinalis): seeds
- Mint (Mentha spp.): stolons and creeping stems
- Monarde (Monarda didyma): rhizomes
- Mustard (Brassica nigra and B. juncea): seeds
- Origan (Origanum vulgare): seeds
- Russian tarragon (Artemisia dracunculus): rhizomes and seeds
- Shisho or perilla (Perilla frutescens): seeds
- Sweet cicely (Myrrhis odorata): seeds
- Sweet woodruff (Galium odoratum): rhizomes
- Tansy (Tanacetum vulgare): rhizomes and seeds
- Thyme (Thymus vulgaris): seeds
- Valerian (Valeriana officinalis): seeds
How to Control Weedy Herbs
Weedy or not, several of the herbs presented above are essential to any herb garden. Can you even imagine cooking without thyme, oregano or chives? But fortunately there are ways to grow weedy herbs while limiting their ability to invade. Here are a few:
A. Self-Sowing Herbs
- Either remove all their flowers or harvest them before any seeds ripen;
- Apply 3 to 4 inches (7 to 10 cm) of your choice of organic mulch (shredded leaves, wood chips, forestry mulch, etc.) throughout the herb garden, completely covering the soil. Seeds will not germinate in mulch-covered soil;
- Hand pull when plants are still small;
- Grow them beyond their hardiness zone. For example, fennel is hardy from zone 6 to 9 and can be weedy there if you let it go to seed. However, it won’t be invasive in zones 1 to 5.
B. Herbs With Wandering Rhizomes and Stolons
- Cultivate them in pots on a deck, patio or balcony: that will nip any spread in the bud;
- Plant them inside a barrier sunk into the ground. This could simply be a plastic pot or pail with its bottom removed. The barrier should stick up at least 2 inches (5 cm) above the ground as the rhizomes of some plants, such as mint, right will creep right over a barrier that is level with the ground.
Don’t hesitate to grow herbs: most are great and very productive plants and you’ll be thrilled with the results. But do take note of the invasive ones. After all, forewarned is forearmed!