Perennial plants are a gardener’s best friend since they return year after year. While you may think perennials are mostly flowers, there are actually many perennial vegetables and herbs. Since they’re going to stick around in the garden for a while, you need to know which ones to keep apart. This is also true of annuals that don’t like growing with certain perennials.
Not all plants are neighborly to each other. Some can inhibit growth, while others grow too viciously and overpower their neighbors. Drought-tolerant plants and moisture-loving plants don’t work well together, either.
There are lots of reasons why perennials won’t share a space for more than a season. Let’s take a look at a few pairs that are totally toxic roommates!
Fennel and Kohlrabi
Fennel isn’t friendly to most edible plants thanks to its allelopathy. An allelopathic plant creates chemicals that stunt other plants or stops their growth altogether. Talk about rude neighbors!
Fennel and kohlrabi are perhaps the least agreeable pair. Not only will fennel prevent kohlrabi from growing, but they’ll also take up too much root space and compete for space and nutrients. They both have leafy tops and large bulbs, so overcrowding is bound to happen.
Gardenias and More Gardenias
Gardenias have so many beautiful color options to choose from that you’ll want to plant them all! Unfortunately, that’s not the best idea. Although lovely, they can spread diseases pretty quickly, and nothing will pick up a gardenia disease like a gardenia!
Though you may have an image of beautiful gardenia hedges surrounding your garden, you may need to switch up your plans. Plant shallow-rooted plants that like shade between your gardenias to help prevent the spread of diseases. Hostas, boxwoods, and wax begonias are all great options that will look stunning.
Garlic and Onions
Garlic and onions are typically grown as annuals, but there are several perennial varieties that will keep your garden aromatic for years to come. If you choose to grow perennial garlic and onions, don’t keep them too close.
Both are in the allium family and share the same problems like pink root, Fusarium bulb rot, wireworms, and onion maggots. If one plant comes down with a disease or pest, it could infect the other. Garlic has better plants to companion plant with, and onions have several better pairings as well.
The pests and diseases are likely to stay in the area if perennials are around. Keep them apart and grow suitable companion plants that will keep pests at bay.
Lavender and Hostas
The dainty purple hues of lavender and bold green foliage of hostas would make a beautiful landscape in theory, but in practice, it’s a bit of a nightmare to keep everyone happy. Lavender is made of tough stuff and prefers lots of sun and slightly dry soil. Hostas, however, prefer shade and don’t like to go thirsty. If you grow them together, at least one of them will be unhappy!
Grow lavender with other drought-tolerant plants like yarrow, basil, or echinacea. The plants will be happy, and you’ll save yourself a headache.
Marigolds and Beans
Many marigolds are annuals, but there are some perennial varieties that will come back each year, like the Mexican marigold (Tagetes lucida). Marigolds are usually touted as a cure-all for garden pests. Got mosquitoes, aphids, or squash bugs? Bring out the marigolds!
But these little fighters have a few enemies they’re not so good at battling, like spider mites, for instance. If you grow marigolds next to something else that spider mites enjoy, like beans, you risk having a big problem! Keep spider mite-prone plants away from each other so you can keep those little bugs at bay.
Milkweed and Forget-Me-Nots
Milkweed is an important plant for Monarch butterflies, as it’s the only thing the larvae will eat. While you may want to sprinkle some milkweed into your flower beds, make sure you aren’t growing incompatible plants there.
Milkweed likes sandy and dry soil, lots of sunshine, and doesn’t need much water to be happy. Pair them with forget-me-nots, and you’ll forget you ever planted them! If you prioritize the happiness of your milkweed, most other flowers will dry out.
Mint and Strawberries
I could go for a strawberry lemonade adorned by some fresh mint right about now! But I wouldn’t dare try to grow mint and strawberries together. Both plants spread like weeds in the right conditions and will crowd each other out in no time.
Both plants will risk getting stunted, but if I had to guess, I’d say the strawberries would lose this battle. I once ignored everyone’s advice and planted mint in my raised bed and soon had mint on the other side. I could still smell it the following year, even though I thought I completely removed it!
Oregano and Basil
No herb garden is complete without oregano and basil, but you’ll want to keep these fellas in separate containers. Like other pairings I’ve mentioned, these two aren’t compatible due to water needs. Oregano is more drought-tolerant than basil and may get overwatered as you try to keep up with the basil’s needs.
Basil and oregano can also compete for root space since they have shallow root systems. The leafy parts of the plants shouldn’t have any issues being nearby, so plant them in containers next to each other so they can be together but apart.
Rosemary and Cucumbers
Cucumbers are super thirsty and need lots of water, but rosemary, not so much. Rosemary can be drought-tolerant and survive on infrequent watering. Cucumbers won’t put up with that kind of treatment, though!
Pair rosemary with oregano since they like similar conditions, and save the cucumbers for a moist part of your garden.
Tansy and Almost Everything
Tansy can be a bit of a problem, but just because it’s aggressive doesn’t mean it should be shunned completely. Plant tansy with care, making sure you don’t plant it in areas where it’s listed as a noxious weed. If you plant it, keep it in a container and deadhead the flowers before they go to seed so you can keep control of the population.
This aggressive grower can choke out surrounding plants, which is why I say you shouldn’t plant it with almost everything! It can grow up to 4 feet tall and spread out to almost 2 feet, so short plants don’t stand a chance against it.
Shade Trees and Grass
If you have a patchy lawn, then you know it’s no secret that turfgrass and shady trees aren’t friends. Most grasses used for lawns thrive in the sun, so a large tree will result in dirt patches under the shade.
It’s not the worst thing in the world since the lack of grass will give you some space to mulch your tree to provide even moisture. But you won’t have much luck if you’re after a pristine lawn with a shade tree.
Walnuts and Many Plants
Earlier, I mentioned allelopathic plants and walnut trees are prime examples, with black walnuts being the worst. They inhibit the growth of several plants, from grass to flowers to edible plants.
Some plants are tolerant to juglone, the compound in the tree that causes problems. The tolerant plants are, of course, other allelopathic plants with juglone! Many trees like pawpaw, hickory, and sycamore can grow next to walnuts, along with viburnum, forsythia, and black raspberry bushes.
If you want to grow edible plants, try onions, melons, beans, and squash, but be wary of how much shade is cast on these. Some flowers, such as calendulas, marigolds, and zinnias, are also tolerant of juglone.
Yarrow and Bee Balm
Yarrow is similar to tansy in that it can become aggressive in the right conditions. Due to its tolerance to heat and drought, it will likely find a home wherever you plant it and proceed to grow vigorously.
Bee balm is a plant that won’t get along with yarrow. It likes moist soil, and though it can grow a bit taller than a yarrow, it likely won’t be able to keep up with the yarrow’s spread and will eventually get choked out.
Final Thoughts
You can count on your perennials returning next year if you give them good neighbors. Don’t worry too much about memorizing which plant goes with which—it’s all about their growth requirements and keeping similar plants together.
