Keep these plants away from each other

By Diane Beyer

Everyone has had an experience with a bad neighbor. There are various reasons for considering a neighbor “bad,” but most of them have an element of “chemistry” in them somewhere. Some people just don’t get along. It’s no different in the plant world. Since plants are restricted in place and not able to move away from bad or undesirable neighbors, they must employ other methods. Plant communities use chemistry to repel or subdue those that may pose a threat to a thriving population. 

Planting fennel near peppers may cause stunted fruits. The peppers are edible, just not pretty. Photo courtesy of Diane Beyer.

There are several other things to consider when deciding what to plant with what:

• What growing conditions do the plants require? Plants that have different requirements for soil pH level, sun exposure, nutrients, or moisture should not be planted together and expected to thrive. Plants also release varying amounts of nutrients, such as nitrogen and potassium, which can affect the growing conditions of nearby plants by altering proximate pH levels.

• In contrast, do not plant two heavy feeders together, as one will inevitably bully the other by sucking up nutrients needed by both. 

• How tall do the plants get? This should be taken into account when planting small annuals or large trees. If plants, such as tomatoes, are planted next to sun-loving plants, such as bush beans, chances are good that the beans will suffer from the shade of the taller, more aggressive tomatoes. It is possible to plant plants of varying heights together if the smaller ones are oriented such that they receive the most sun exposure.

• Plants that require large amounts of water will scavenge water from surrounding soils, harming nearby plants that are not as effective at water consumption. 

• Plants that attract similar pests and/or diseases benefit from not being planted in close proximity.

Although sunflowers (Helianthus annuus) may affect the growth some veggies, they do provide an excellent nectar source and also distract birds from other plants and fruits. They may also help repel aphids. Photo courtesy of Jeanne Grunert.

While the list above is a good start, there is a more complex consideration to be made and it involves chemistry. Many plants release compounds called allelochemicals that repel or inhibit growth of other plant life. Some allelochemicals are non-selective, killing or repelling most other plant life in an effort to maintain survival of the species. Other allelochemicals are more selective, seeming to only inhibit plant life that may compete for the same nutrients, light, or moisture. This biological phenomenon is called allelopathy. 

If you have issues with plants growing near each other and all other factors check out, there may be chemical warfare afoot.

The word allelopathy comes from two Greek words: allelon, which means “of each other” and pathos, which means, “to suffer.” Plants can employ allelopathic tactics in various ways. Some plants contain allelochemicals in their leaves, thus enabling them to repel and inhibit nearby growth through gases expelled through transpiration or through the decomposition of dropped leaves. Other plants contain the repelling chemicals in their roots. As these toxins are released into the soil, roots or nearby plants absorb them and are stunted or killed. Allelochemicals can also affect or hinder seed germination. When used correctly, these chemicals can be a very effective method of weed control. 

Studies have been able to show the effectiveness of this chemical warfare in some species. For example, the aggressive, invasive tree of heaven (Ailanthus altissima) produces ailanthone, a chemical that exhibits non-selective, post-emergent herbicidal activity similar to glyphosate. This explains why, when you see “groves” of tree of heaven along a highway or under a power line, there is NOTHING else growing in that grove. Eventually, is left unchecked, they will overcome any other vegetation in the area, including native trees.  

Another example, black walnut tree (Juglans nigra) exudes a chemical compound called juglone from the leaves, stems, and roots. Juglone can damage or even kill susceptible plants, such as solanaceous crops. 

Black walnut trees produce juglone to inhibit the growth many plants. Plants most vulnerable to the black walnut’s toxicity include nightshades such as tomatoes, peppers, eggplants, and potatoes. Others easily tolerate juglone, including melons, beans and carrots. Photo courtesy of Diane Beyer.

So how does all this affect your vegetable garden? There has been much ado about companion planting, but not much about what NOT to plant together. Some of the following is based on science, some on anecdotal evidence – regardless, it might be worth a shot!

Carrots: Parsnips and carrots are both susceptible to the same soil-borne diseases and carrot flies so it’s best to plant them away from each other.

Tomatoes: Tomatoes do not like brassicas (cabbage, broccoli, kale, etc.) as they become stunted and crop production is reduced. Brassicas contain glucosinolates, sulfur-containing phytochemicals that are possibly offensive and detrimental to the tomato plants. Even though studies have shown that consumption of brassicas could reduce the risk for multiple types of cancer in humans, tomatoes are not impressed. It is also not advisable to plant corn near tomatoes, as the two plants are prone to similar fungal diseases and may attract the same insects. 

Potatoes: Potatoes are somewhat of a garden thug in that they make many enemies. It’s probably a chemical thing. Don’t plant these tasty tubers near asparagus, brassicas, carrots, cucumbers, eggplant, melons, peppers, squash, tomatoes or sunflowers! 

Although potatoes can be thugs in the garden, they also benefit from and help out a few friends. Green beans can help by these tubers by repelling the Colorado potato beetle, and they don’t compete with potatoes for nutrients. In fact, beans enrich the soil with nitrogen, which improves the growth of potatoes and adds resiliency. Potatoes reciprocate by protecting green beans from Mexican beetles. Photo courtesy of Diane Beyer.

Beans: Avoid planting beans near chives, garlic, onions and leeks. Pole beans can also stunt and be stunted by beets, but bush beans seem to do fine. 

Brassicas: Avoid planting veggies from this family near potatoes, tomatoes, peppers, or eggplant, as these four plants like a fairly acidic soil pH around 5.5 to 6.5 whereas broccoli, cabbages, and kales like a more neutral soil. 

Squash: Avoid planting near brassicas and potatoes, as they have different growing requirements, and potatoes may employ chemical warfare!

Sunflowers: Although sunflowers provide lots of high protein nectar for pollinators, they also contain allelopathic chemicals that are especially effective on beans and potatoes. Planted around a garden, sunflowers can serve to keep weeds down and also draw well-needed pollinators.

On a positive note, spinach gets along with everyone! You’ll still have to keep in mind that most spinach is shorter than other veggie tops and excessive shading will be detrimental. 

Lettuce and spinach are friendly neighbors and get along with almost everyone. Be careful not to shade these early garden goodies with taller plants. Photo courtesy of Barbara Pleasant.

Some vegetables also inhibit cropping of the same species in the same location from year to year. Disease and pest prone broccoli is one of these and the probable reason it will leave behind a deterrent residue after being harvested is to aid in preventing disease and pathogens from overwintering and attacking early spring crops of the same type. Thus, the need for crop rotation. 

Some diseases require two or more hosts to complete their life cycle. Cedar-apple rust is one of these, so planting the eastern red cedar (Juniperus virginiana) near your apple orchard is not a good idea, as the rust disease will infect the fruits causing spotting and possible fruit drop. Photo courtesy of Diane Beyer.

In the general landscape, be aware that there are other plants containing allelochemicals. If you have issues with plants growing near each other and all other factors check out, there may be chemical warfare afoot. Here are some common offenders:

• Aster and goldenrod (Solidago spp.) inhibit sugar maples (Acer saccharum) and tulip poplar (Liriodendron tulipifera).

Kentucky bluegrass will inhibit Forsythia and black cherries (Prunus serotina).

Junipers (Juniperus spp.) inhibit grasses. 

Don’t let any of this scare you away from trying different combinations. Good, healthy soil with lots of organisms can break down the toxins into more benign elements. And a well-draining soil may allow the toxins to move below the root zones of nearby plants. On the other hand, some soil microbes can assist with allelopathy. That’s how it goes with neighbors.

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