Garlic is one of the most popular companion plants. It can be grown next to most plants as a natural pest and fungus deterrent. It takes up little space, is not fussy about soil and can grow in most conditions.
I am sure that its pungent flavor is what convinces people that it keeps pests and diseases away. If it keeps vampires away, surely a few bugs are not a problem for it.
Garlic, the King of Companion Planting
What are the claims for garlic (Allium sativum)?
Garlic deters pests, including fungus gnats, codling moths, spider mites, cabbage loppers, Japanese beetles and even ants. The pungent flavor of garlic is due to an accumulation of sulfur compounds which are natural fungicides. That explains its ability to ward off disease.
Garlic gets along with most plants, but it should not be grown near asparagus, peas, beans, sage, parsley and strawberries, because it will stunt their growth.
Does Garlic Deter Pests?
A study in Brazil planted garlic, chives, coriander, fennel, oregano, or sweet marjoram in fields of strawberries. They then counted the number of two-spotted spider mites. Tests were done in both the field and in greenhouse settings. Garlic caused a greater reduction (up to 52 %) in strawberry plants when higher populations of two-spotted spider mites occurred in the field. Fennel and chives also showed some reduction.
It is important to note that the pest was not eliminated, and worked best only when pest populations were high.
An interesting study in Zimbabwe compared cabbage planted with garlic to cabbage sprayed with malathion 25WP. They were checking for the diamondback moth. There was no significant difference between the two groups, but counts were higher in a control with no spray or garlic.
A study in China looked at the green peach aphid in tobacco and found numbers were lower with garlic planted, “especially when populations peaked. Other arthropod populations were not negatively affected by intercropping garlic. Species richness, diversity, and stability of the arthropod communities increased.” This seems suspicious. Why was only one arthropod affected?
This study from Botswana intercropped kale with either garlic, basil or marigolds. Basil had the least number of cabbage aphids, but garlic and marigold also reduced the aphid.
A UK study showed that flea beetles laid as many eggs on cauliflowers with or without mint, garlic, dill or sage.
Summary for Pests
These are just some of the studies that have been done. Garlic does seem to reduce certain pests. It has been suggested that garlic releases volatile oils which may confuse flying insects, making it more difficult for them to find their host plant.
If you compare the claimed list of pests with the ones tested in these reports, there is not much correlation. I suspect the lists you find in most gardening sources for companion planting have been made up and repeated so many times that now people believe them.
In order to reach any conclusion about a certain insect, it needs to be tested on the host plant of interest along with garlic.
What happens to beneficial insects such as pollinators and predator insects? If garlic keeps pest insects away it will also keep beneficial insects away. What effect does that have on crops?
Note: A comment was left at our Facebook page when this article was discussed: “My Garlic Farm used to have Aphid outbreaks.”
Does Garlic Increase Yield?
It might repel pests, but does it increase yield. As discussed previously, this is the important question that needs to be asked of any companion planting recommendation.
A study in China intercropped eggplant with garlic and found a slight (2-6%) yield increase (kilogram per hectare). It is not clear if this was statistically significant.
Testing in India showed that garlic planted with sugar cane reduced the sugar cane crop, but the total value of both crops increased.
Research in Egypt showed that strawberry crop yields were reduced when combined with garlic, peppers or snap beans. Strawberries are one crop that is not recommended with garlic.
The evidence that garlic increases yields in not strong. One can assume that if pest damage is reduced, yields go up, but the scientific data to confirm this is limited.
Can Garlic be Planted with Beans?
Apparently these two plants should not be grown together because garlic stunts the growth of beans, but research does not support this. This study did not look specifically for this effect, but it reports the two crops exchanging nitrogen with no indication that beans were affected.
Garlic May Affect the Target Crop
How does garlic, grown beside another crop, change that crop?
This study looked at changes of nutrients in cucumber plants grown with and without garlic, in pots. Nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and manganese (Mn) in cucumber plants were significantly increased while magnesium (Mg) levels were lower. The amount of change was relative to the number of garlic bulbs planted with medium planting showing the biggest benefit.
When Chinese cabbage (Brassica chinensis) was grown with and without garlic researches found that cabbage with garlic had higher levels of soluble protein and nitrate, but sugar content was not significantly affected.
Garlic Changes Soil
A study in China looked at changes in the soil, under plastic tunnels, when peppers where grown either by themselves or with garlic. “Results showed that bacteria population significantly increased in the pepper plot intercropped with normal bulb garlic, while actinomycetes were significantly enhanced in pepper plot intercropped with green garlic. Populations of fungi were significantly inhibited in pepper-green garlic intercropped plots.”
The fact that garlic affects the soil microbes is an interesting fact, but on its own it does not tell us very much about the crops. Such changes may improve the pepper crop yield, or it might decrease the yield, depending on which microbes are affected. What we can say is that the garlic does have an effect on the microbe population in the soil.
This research also looked at enzymes, pH and EC changes in the soil, and concluded that ” intercropping peppers with green garlic improved soil microbial and biochemical properties as compared to monocropping.”
The other interesting aspect of this work is that regular bulb garlic and green garlic (different cultivars of Allium sativum) produced different results.
A very similar study looking at both bulb garlic and green garlic cultivars grown with peppers, report the same results. They also found that intercropping actually increased the NPK in the soil, and concluded that “the intercropping of peppers with garlic enhances the soil fertility by changing nutrient levels, enzymatic activity and the soil microbial population.”
What these last two studies show is that any effect on a crop varies with the cultivar that is used. Not all garlic produces the same results. When companion planting is discussed in books and blogs varieties are hardly ever mentioned.
The claim that the sulfur compounds produced by garlic are a natural fungicide, seem to hold up, at least in the soil. That does not mean they reduce disease above ground.
Does Intercropping with Garlic Work?
The story is much more complicated than implied by popular companion planting text and this question can’t be answered unless we know the crops being discussed. For most crops there is little if any research available, but the yield for certain crops is increased when grown with garlic.
There seems to be some clear evidence that garlic does reduce the occurrence of some pests. But none of the studies that looked at pest populations also looked at yields. Does the reduction of a pest produce higher yields? It is not clear from the data I found.
It seems clear that intercropping with garlic changes a number of soil parameters including both nutrient levels and living populations. What is not so clear is how all of this affects plants. How does the increase in bacteria and decrease in fungi affect crops?
The benefits of companion planting can be a one way benefit or a two way benefit. None of the reports that I found suggested that the garlic crop benefits. This may be due to the fact that the researchers were looking for ways to increase more valuable crops and garlic was not important, or garlic did not benefit.
Is Garlic Intercropping Practical?
Most of the existing research is done in countries other than North America and Europe. It is possible that the interest is greater in countries that have less access to pesticides, are less mechanized and have lower manual labor costs.
Keep in mind that garlic is planted in fall, harvested in mid summer, and peppers are planted in spring and harvested in late summer or fall – at least in zone 5. That complicates an automated process.
This is much less of a problem for the home gardener who does everything by hand.
There is also another practical aspect to companion planting. Most crops have a preferred spacing. How does that change if you intercrop? There is little discussion about this in circles promoting companion planting.
From a home owners point of view there probably is some value in intercropping with garlic. But there is very little concrete information available about which crops benefit. The lists found in popular text on the subject are not based on science. Most recommendations seem to be based on an early book, called Carrots Love Tomatoes by Louise Riotte, which is not science based. Additional information such as crop spacing, and best cultivars is completely lacking.
My feeling is that home owners can consider trying garlic if they have a pest problem, but unless they are trying to solve a real problem, garlic may or may not be beneficial and it might even reduce yield on some crops.
Without careful control testing, opinions from home owners on social media are mostly worthless.
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