Incorporating fresh culinary and medicinal herbs is a rewarding experience for many gardeners. However, some popular plants possess a vigorous growth habit that can quickly overwhelm a garden bed. While these species are not typically classified as legally “invasive species,” their aggressive spread necessitates careful management. Understanding the biological mechanisms behind this rapid expansion is the first step in successfully integrating these productive herbs into your landscape.
Understanding Aggressive Growth Patterns
The tendency of certain herbs to colonize space comes down to a few highly successful reproductive strategies. One common method is the development of rhizomes, which are horizontal underground stems that send up new shoots. Plants utilizing this method, such as mint, are difficult to eradicate because even a small fragment of rhizome left in the soil can regenerate. This extensive root network allows the herb to spread rapidly outward from its original planting location.
Another strategy involves prolific self-seeding, where plants produce a large quantity of viable seed that easily disperses. Herbs like dill and chamomile drop seeds directly below the parent plant, or their lightweight seeds can be carried by wind or water. If the flowers are not consistently removed before they mature, a single plant can result in dozens of unwanted volunteer seedlings the following season. Many perennial herbs also employ this tactic, though it is a common mechanism for annuals and biennials.
A third method of spread involves the production of offsets, tubers, or bulbs. Members of the Allium family, such as chives, form small underground bulbs that continuously divide and multiply, resulting in an ever-widening clump. Similarly, plants like horseradish develop deep, extensive, and brittle root systems. If a section of this root is broken or left behind during weeding, it can sprout a new plant. This makes physical removal challenging, as the plant regenerates from underground storage organs.
The Most Common Spreading Herbs
The mint family (Lamiaceae) contains some of the most aggressive herbs, primarily due to their reliance on rhizomatous spread. Peppermint, spearmint, and ornamental varieties like chocolate mint, quickly send out runners that can travel several feet in a single season. These runners form dense mats beneath the soil surface, choking out neighboring plants. Once established, the density of the root mass makes mint challenging to remove completely from an open garden bed.
Oregano (Origanum vulgare) is another member of the mint family that combines runners with a tendency to self-seed. The plant forms a low, bushy mound that expands laterally each year as its underground stems creep into new territory. If the flowers are allowed to mature, they will scatter seeds, contributing to its dual method of colonization. Gardeners must manage both the spreading roots and the potential for new seedlings to contain its spread.
Lemon Balm (Melissa officinalis) is a lemon-scented herb and an aggressive self-seeder that can quickly become a nuisance. Its seeds remain viable in the soil for a long time, leading to unexpected seedlings popping up across the garden years after the original plant was removed. While it spreads by shallow roots, its most problematic trait is the volume of volunteer plants it produces. Deadheading the flowers before they set seed is the primary defense against its spread.
Horseradish (Armoracia rusticana) is aggressive due to its deep, expansive, and brittle root system. When the main root is harvested or disturbed, small pieces break off easily, and each fragment can sprout a new plant. This regenerative ability means that attempting to dig up an established horseradish plant often results in propagating dozens of new plants. Its roots can extend several feet deep, making manual removal a monumental task.
Comfrey (Symphytum officinale) is a medicinal herb known for its extensive, vigorous taproot and ability to self-seed. The plant develops a deep, thick root mass that is extremely difficult to pull out entirely. Like horseradish, any small section of the root left behind can sprout a new plant. Its flower stalks produce numerous seeds that drop and germinate easily. It requires substantial dedication to keep it confined to a single spot.
Dill (Anethum graveolens) and Fennel (Foeniculum vulgare) are prolific self-seeders that can take over a vegetable patch. Although typically grown as annuals or tender perennials, they produce large, umbrella-like flower heads packed with seeds. Once the seeds mature and drop, numerous seedlings appear the following spring, often popping up in unexpected places like pathways or pavement cracks. Managing their spread requires vigilance and the removal of flower heads before they dry out.
Practical Methods for Garden Containment
The most secure method for containing herbs that spread via underground runners is container gardening. Planting mint, oregano, and lemon balm in pots restricts their rhizomes from entering the surrounding soil. For cold climates, these pots can be sunk directly into the garden bed, providing insulation while creating an impenetrable root barrier.
If growing in the ground is preferred, installing a root barrier is an effective physical control measure. This involves sinking a non-porous material, such as metal edging or thick plastic, at least 10 to 18 inches deep around the planting area. The barrier should extend an inch or two above the soil line, as some runners, particularly mint, will creep over a barrier that is flush with the ground.
Strategic pruning and deadheading are the best defense against herbs that spread through self-seeding. Regularly clipping off flower heads from plants like dill, chamomile, and lemon balm prevents them from completing their reproductive cycle. This action should be taken as soon as the flowers begin to fade, before the seeds mature and scatter.
For herbs that form dense clumps or spread by offsets, such as chives, regular division is necessary to maintain control. Every two to four years, the entire clump should be dug up and separated, with the outer, younger sections replanted. This practice manages the size of the plant and revitalizes the herb, ensuring continued vigorous growth in the desired location.