Companion planting suggests certain species thrive together, but some pairings are detrimental. The perennial herb Nepeta cataria, or catnip, requires careful placement. Its vigorous nature and chemical output create a hostile environment for many garden plants. This incompatibility stems from physical resource domination and chemical growth inhibition.
Plants Inhibited by Catnip’s Aggressive Growth
Catnip is a member of the mint family, sharing its tendency toward aggressive horizontal expansion. This vigorous herb spreads primarily through underground runners (rhizomes), quickly colonizing the surrounding soil. These runners establish a dense, competitive root network that efficiently captures water and nutrients before slower-growing neighbors can access them. Catnip’s robust growth physically overwhelms plants that are small, delicate, or slow to establish.
Small, low-lying herbs are particularly vulnerable to being smothered by catnip’s dense foliage canopy. Herbs like chamomile, small thyme varieties, and dill struggle to compete for necessary sunlight when planted too closely. Catnip is also a prolific self-seeder, producing numerous seeds that readily germinate in disturbed soil. This aggressive self-seeding allows new catnip seedlings to choke out valuable space used by desirable plants.
Any plant relying on a slow growth cycle or requiring ample air circulation faces significant disadvantages when competing with catnip. The sheer physical bulk of a mature plant can shade out and suppress low-growing ground covers or young vegetable seedlings. Gardeners aiming for diverse or tightly spaced planting must actively manage catnip to prevent resource monopolization.
Crop Families Sensitive to Catnip’s Chemistry
Beyond physical competition, catnip exhibits allelopathy, releasing compounds that inhibit the germination or growth of neighboring plants. Studies confirm that Nepeta species produce essential oils and root exudates with phytotoxic properties. These chemicals, including monoterpenes and volatile compounds like nepetalactone, subtly undermine nearby crops.
Chemical inhibition is particularly problematic for delicate leafy greens and young seedlings. Laboratory tests using catnip extracts show clear inhibitory effects on the germination and root development of sensitive species like lettuce. Volatile compounds released from catnip foliage can induce seed dormancy, preventing susceptible partners from establishing a foothold in the soil.
Members of the Brassica family (cabbage, broccoli, and kale) are also sensitive to allelochemicals and should be kept separate from catnip. Testing on curly cress, a fast-growing Brassica, demonstrated that catnip extracts inhibit its shoot and root development. The altered soil chemistry surrounding a mature catnip plant can reduce overall crop yield for sensitive plants, even without physical root competition.
Containing Catnip to Prevent Incompatibility
Gardeners can still enjoy catnip’s benefits, such as its insect-repelling qualities, by implementing strict containment strategies. The most effective method is growing catnip in containers, which physically restricts both its rhizomatous spread and root exudates from mixing with garden soil. Sinking the pot into the ground maintains the necessary barrier while giving the appearance of in-ground planting.
For in-ground planting, installing a physical barrier, such as plastic or metal edging extending at least ten inches deep, prevents underground runners from escaping. Managing self-seeding requires regular deadheading of flowers as soon as they fade. Removing spent blooms before they set seed prevents the scattering of unwanted catnip seedlings. A strict spatial separation of two to three feet between catnip and sensitive crops provides a buffer zone against physical and chemical influence.