The genus Allium encompasses a wide variety of plants, from common culinary staples like garlic, onions, and chives to dramatic, globe-shaped ornamental flowers. Companion planting involves growing different species in close proximity to benefit one or both plants, often by deterring pests or improving growth. Alliums are highly valued in this practice due to their strong presence and unique chemical properties, making them beneficial partners in both the vegetable garden and the ornamental border. Finding the right planting partners can significantly improve the health and appearance of the garden landscape.
The Role of Allium in Companion Planting
The power of Allium species as garden partners comes from their distinctive aroma, which is a natural defense mechanism against many garden threats. These plants produce volatile sulfur-containing compounds, such as allicin and diallyl polysulfides, when their tissues are damaged or when their roots exude chemicals into the soil. This pungent chemistry acts as a broad-spectrum deterrent against various insects, fungi, and microscopic pests.
The sulfur compounds released by Alliums possess insecticidal, fungicidal, and nematicidal properties, confusing or repelling pests rather than killing them outright. This strong odor effectively masks the scent of nearby vulnerable plants, preventing pests like the carrot root fly from locating their preferred hosts. Growing Alliums near other crops creates a natural chemical barrier in the soil and air, contributing to a healthier, more balanced garden ecosystem.
Ideal Vegetable and Herb Companions
Alliums are particularly beneficial when interplanted with various vegetables susceptible to common garden pests. Tomatoes, peppers, and eggplants, for instance, benefit from the proximity of garlic or onions, which helps keep certain insect populations at bay.
Brassicas, including cabbage, broccoli, and cauliflower, are frequently targeted by pests like the cabbage worm, aphids, and spider mites. Intercropping these plants with chives or garlic significantly reduces the prevalence of these damaging insects and can lead to improved yields. The strong scent acts as a repellent, preventing pests from settling and laying eggs on the brassica foliage.
Carrots also benefit greatly from this partnership, as planting Alliums nearby confuses the female carrot rust fly, which relies on scent to find the carrot roots for egg-laying. Additionally, planting garlic or chives near fruit trees and rose bushes helps deter destructive insects like aphids and weevils. This protective pairing can also provide a fungicidal benefit, helping to protect roses from diseases like black spot and mildew.
Aesthetic and Structural Flower Pairings
Ornamental Alliums, with their striking spherical blooms on tall, slender stalks, are often paired with other flowers to solve a specific aesthetic problem. Many large-headed varieties, such as ‘Purple Sensation’ or ‘Globemaster,’ have foliage that naturally begins to yellow and die back while the flower is still blooming. Since the leaves are necessary for the bulb to store energy for the following year, they should not be removed prematurely.
To conceal this unsightly fading foliage, plant the Alliums among lower-growing, leafy perennials that emerge later in the season. These plants offer dense, fresh foliage that effectively hides the yellowing Allium leaves as they mature. Low-growing annuals can also serve this purpose, providing a clean ground cover.
For continuous visual interest, choose perennials that bloom slightly later or offer a contrasting shape to the Allium’s globe. Herbaceous Peonies and Baptisia provide a solid structure and different flower form to complement the spheres. Spire-shaped flowers, such as Salvia, Verbascum, and Foxglove, create an appealing juxtaposition of form when planted alongside the round Allium heads.
Foliage Concealment Options
- Hostas
- Lady’s Mantle
- Cranesbill Geraniums
- Euphorbia
- Petunias
- Calendula
Plants to Avoid Planting Near Allium
Not all plants are compatible with the potent chemistry of Alliums, and planting them too close can lead to stunted growth or reduced yields. The most significant incompatibility is with legumes, which include beans and peas. Alliums exhibit a form of chemical interference known as allelopathy, releasing sulfur compounds that inhibit the growth of nearby plants.
In the case of legumes, the sulfur compounds from Alliums disrupt the nitrogen-fixing bacteria, called rhizobia, that colonize the legume roots. Since legumes rely on these bacteria to convert atmospheric nitrogen into a usable nutrient form, the interference can severely stunt their growth and compromise the harvest. It is best to maintain a clear separation between Alliums and all types of beans and peas in the garden.
Certain other vegetables and herbs should also be avoided due to resource competition or shared vulnerabilities. Asparagus, for example, is not a suitable partner, as it can be stunted by the allelopathic chemicals and competition from aggressive Allium roots. Some root crops, like turnips, may also suffer inhibited growth and develop a less desirable flavor when planted in close proximity to onions or garlic.