Companion Planting

With the 2025 growing season on the not-so-distant horizon, many avid home gardeners are currently hard at work choosing and ordering the seeds they will soon start indoors. However, it may not just be production plants they’re aiming to sow. With the many benefits they can provide for a garden, companion plants may also be finding their way onto the sowing list. But what truly constitutes a “companion plant?” And, perhaps more importantly, what differentiates reality from myth regarding their benefits?

In recent years, the term “companion planting” has unfortunately become synonymous with non-research-based tips and tricks. Suggestions such as ‘plant tomatoes with carrots to increase yields,’ plant garlic to repel all pests,’ and ‘plant marigolds to protect your entire garden’ have become widespread, due in large part to unchecked social media. However, the truth to these tips usually lie somewhere between fact and fiction.

While carrots and tomatoes can grow together, evidence shows they don’t care much about each other being there, nor are they responding. While the sulfur compounds within garlic do repel some pests, this effect is centralized to the garlic itself –and only with certain pests. While the citrus oils within marigolds can offer some protection, particularly with nematodes, this protection is still limited and is not an all-encompassing fix-all.

This is not to say, however, that companion planting is without merit. The practice itself has benefited fruit and vegetable growers for thousands of years, beginning with the “Three Sisters Method” in indigenous cultures, proliferating through the Middle Ages, and re-popularized in the 20th century with the organic food movement.

So, what works?

Building on the Three Sisters Method, some plants, like legumes, can act as a naturally regenerating fertilizer source by enhancing soil fertility and improving conditions for nearby crops. Some plants, like squash, can act as natural weed control, stifling weed growth and eliminating their “pest hotel” and plant competition qualities. And then some plants, like corn, can offer a more practical benefit, acting as a literal trellis for others.

Elsewhere, companion planting can be utilized for pest control, with the understanding that this control will not be all-encompassing. In addition to marigolds deterring nematodes, other aromatic plants such as basil and rosemary have been shown to repel pests like aphids and whiteflies. Different plants, like nasturtiums, act as effective trap crops by being the more desirable plant for common garden pests –leaving the trap plant to be treated or removed over the one producing food.

All in all, the benefits of companion plants really boil down to the individual growing characteristics of the plants incorporated, how each of those plants interacts with their growing environment, and how other plants can utilize those characteristics and interactions. Bear in mind, however, just as some plant combinations can benefit each other, different combinations can antagonize each other just as well. Due diligence in research, and perhaps a few at-home trials, will ultimately spell your success.

by Anthony Reardon, Horticulture Small Farms Agent, 2024

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