I have written about companion planting several times and have concluded that most recommended companions, either don’t work, or there is no scientific support for them. When the book, “Plant Partners: Science-Based Companion Planting Strategies for the Vegetable Garden” was released I was definitely intrigued because all prior books on the subject are mostly myths and definitely not based on science.
Jessica Walliser, the author, was kind enough to provide a copy for review. Does this book finally provide a sound set of recommendations for companion planting?
Review of Plant Partners
The title of the book is Plant Partners and not Companion Planting. That is a suitable title since the book explores more than just companion planting, including topics like weed management, cover crops and attracting pollinators. The information is presented in an easy to understand style that is very informative and enjoyable to read. It is sciency, written for the non-scientist.
The chapter on cover crops looks at how you can use plants to enrich the soil while your main crop is not growing. I found the information in this chapter to be very good and science based. Cover crops are not examples of companion planting since the two crops do not grow together, but it can be a useful technique for developing soil.
The book will give you good insight into the concepts of companion planting and some of the general science behind it. It will also help you better understand the garden ecosystem and how the many elements are tied together.
Companion planting is defined in the book as pairing two or more plants for the purpose of enhancing growth and production, or trapping or deterring pests. I would disagree with “or trapping or deterring pests”. Companion planting is not successful just because it traps or deters pests. It must also increase productivity.
The book is strong in the area of pests, which reflects the authors own background.
What About Companion Planting?
The book has several chapters that at first glance don’t seem to have anything to do with companion planting, like the chapter on weed control, but most chapters include some plant partnership recommendations. The main text does not include references or numbers corresponding to references. The scientific references are all found at the back of the book, organized by chapter. Within each chapter, they are listed alphabetically by author. This is unfortunate because it makes it almost impossible to find the scientific support for a claimed companion partnership.
The goal of this review is to ask the question, are the examples of companion planting supported by science?
There are too many combinations listed in the book for me to check each one, but as it turns out, I listened to a popular podcast, A Way to Garden, hosted by Margaret Roach, where she interviewed Jessica Walliser about her new book. In that interview, Jessica gave three examples of plant partners that were “well supported by research”. Great. I might as well review these.
These are the combinations I looked at.
- nasturtiums for the squash bug (page 110)
- basil with tomatoes to control thrips (page 112)
- sweet alyssum grown with lettuce for aphids (156)
I looked up all three combinations in Plant Partners and then I checked all of the references in the associated chapters.
Most of these references looked at plant-insect interactions from a very generic point of view. For examples, insect populations increased or decreased in the presence of certain plants. Many of the references were summary reviews of other studies. I found almost no studies that looked at specific plant combinations.
I did not find a single study that looked at any of the above three plant combinations, either in the lab or in the field.
I contacted Jessica, explained my findings, and asked if she could provide the references that support these combinations. To date I have not received them.
I then did my own search for specific studies that might not have made it into the book. I found some for the sweet alyssum – lettuce combination but none for the other two cases.
Does Sweet Alyssum Reduce Aphid Numbers?
I did find a reference that mentions sweet alyssum, in a different chapter of the book, and it is a good example of the “generic-type” of reviews I mentioned above.
This review talks about using hoverflies (Syrphidae) to control aphids. Most of the article is about their behavior and description of various species but near the end it does mention that plants like sweet alyssum, coriander, barley, and sweet fennel are interplanted with lettuce. These flowering plants attract syrphid flies, which in turn eat aphids, keeping the aphid population down on lettuce. They even include pictures of some organic farms that use this technique.
This seems like strong evidence for this method, but the review goes on to say, “Some insectary plantings (i.e. sweet alyssum) may also harbor pests or result in weed problems. Habitat manipulations also have an economic cost. The land devoted to insectary plantings is lost to cash crops, and this may amount to 10 percent of the arable acreage. There are also costs incurred in planting and maintaining insectary plants.”
They conclude with “In the absence of formal studies, it is still uncertain whether this opportunity cost is offset by improved pest control.”
Based on this reference in Plant Partners, there is no scientific support for using sweet alyssum to keep aphids away from lettuce.
This is a very common problem with companion planting rules. A small part of the full story is used to justify companion planting (i.e. hoverflies like sweet alyssum and eat aphids), and a big part of the story is ignored (extra costs, loss of productive land, introduction of new pests, etc.). A complete scientific study would take all of these factors into account and compare productive yield or value of crops with and without sweet alyssum.
I found another study that concluded, “when coriander was sown around a field of winter wheat in an attempt to influence syrphid distribution between fields, it failed to significantly increase syrphid diversity or species numbers within that field”. Coriander does not seem to work either, at least for winter wheat.
Another study showed a reduction of aphids from hoverflies, when sweet alyssum was grown with lettuce, but it was done using special cages and they clearly state that, “It should be noted, however, that the absolute impact of hoverflies on aphids was not assessed in this study, as a no-hoverfly control treatment was not included.” Translation – it might work, but the science was not done in this study to support such a conclusion.
There is some indication that sweet alyssum attracts hoverflies, and that hoverflies do reduce aphid populations. I found no study that concluded this was a good companion partnership which would increase yield. It may be a good combination where the aphid population is so high that a crop can’t be grown. Any yield is better than zero yield.
More Companion Partners
I thought I would check a couple of other suggested plant partners and see what the references in the book say. I randomly picked two from chapter two.
Cowpeas + Peppers (page 40)
From Plant Partners, “A California study showed that cowpeas improved production in peppers”.
The only reference in this chapter, dealing with peppers, is a review study about peppers which does list a study that looked at growing cowpeas with peppers. It said “intercropping pepper intrarow with cowpea (Vigna unguiculata) reduced flower thrips (Megalurothrips sjostedti) and cowpea aphids (Aphis craccivora) on cowpea and improved cowpea yield compared with sole-cropped cowpea. However, pepper yields were not reported.”
The review also lists other studies that looked at growing peppers with various legumes and in most cases there was no increase in pepper yield.
Green Beans + Potatoes (page 38)
From Plant Partners, “A 2010 study found partnering beans with potatoes produced an increased potato tuber size.”
There is only one reference in chapter 2 that is dated 2010 and deals with potatoes. This study looked at crop rotation – 7 different crops (including beans) followed by potatoes. It did not study intercropping beans and potatoes together.
What it found was, “Addition of a cover crop also resulted in increases in bacterial populations and microbial activity and had significant effects on soil microbial characteristics, in addition to slightly improving tuber yield (4% increase).” It went on to say, “However, this study also demonstrated limitations with 2-year rotations, because all rotations resulted in increasing levels of common scab and Verticillium wilt over time.”
Crop rotation increased tuber size slightly (4%), while increasing a major disease in potatoes. Companion planting was not tested. It is hard to see how this data can be used to recommend the bean + potato partnership.
Pole Beans and Corn
This is an example in the book to illustrate how one plant can be used to provide support for another plant. This is part of the classic Three Sisters and I was surprised to see it in the book because I have reviewed this before and found no evidence that this combination results in increased yield. There is no doubt beans can climb on corn, but beans do not provide nitrogen to the corn and the corn does compete with beans for light, nutrients and water resources. Even indigenous people grew the crops separately when they wanted to maximize yield.
What about references? This chapter #4 has no supporting references for the 12 companion partnerships it presents.
Is Plant Partners, Science-Based?
There is no doubt that the book contains a lot of good information about plants and their interaction with other plants, insects and diseases. The general discussions about how insects behave in a garden environment are also very good. You will certainly gain a better understanding of the value of plant diversity and insect behavior.
Are the suggested companion planting combinations science-based?
I can only speak to the ones I reviewed. A companion planting combination only has scientific support if a study shows that at least one partner benefits. There are good examples of companion planting that are supported by science, including the use of garlic with other plants. In such cases there are one or more specific studies that looked at the combination in question and show positive outcomes. I did not find this kind of support for the ones I looked at.
The idea of increasing plant diversity in the garden is a good one and it might help control pests, but we need a lot more research before we can conclude that most companion planting combinations work. The science is not there yet.