Many years ago, when I had a small garden in San Luis Obispo, my poor radishes had a bad case of cabbage root fly. The recommendation given to me was to apply the insecticide Malathion, and I wish I had known then about the wisdom of intercropping. In many cases, when Brassicas, like cabbage, kale, collards, broccoli, etc. are grown in a plot with other plants, even sometimes weeds, the total yield of the plot is higher, than if each crop were grown alone, and the Brassicas are less damaged by pests and diseases. Below are a few examples, from several countries with differing climates, of Brassicas that were intercropped with other vegetables, (sometimes weeds!) with good results.
In the Netherlands, when cabbage (Brassica oleracea) was grown with clover (Trifolium repens/Trifolium subterraneum), pests such as cabbage root fly (the young flies, called maggots, eat themselves into mostly the roots, using hook-like mouthparts) (Delia radicum) and onion thrip, (they pierce and suck the liquid out of the leaves)(Thrips tabaci), were reduced compared to cabbage grown alone (Theunissen, J. et al. 1992). Basically, Brassicas with clovers = less maggots and thrips.
In Cambridge, England, when Brassicas (mustard family) were grown with "clover, spinach, beans (Phaseolus vulgaris) and grass (Lolium spp.)", or taxonomically unrelated crops, damage to Brassicas by the cabbage aphid (Brevicoryne brassicae), and the cabbage root fly/cabbage maggot () was significantly less (Coaker, T.H. 1980). This worked best when the intercrops were planted early enough so that 50-60% of the ground was covered, before the Brassicas were planted as transplants.
In California, collards (Brassica oleracea) intercropped with beans, that were grown in a plot with weeds, especially crucifer weeds, (mostly Brassica campestris ) as compared to a weeded plot, sustained significantly less damage to their leaves by flea beetles (Phyllotreta cruciferae) (Altieri & Gliessman, 1983)
In Africa, growing kale with nightshades (Solanum spp. for example, potatoes or tomatoes) proved to increase yield of kale leaves, compared to just growing kale by itself (Oseko, et. al., 2003). This example used African kale (Brassica carinatus) and nightshades found in Africa ("Solanum scabrum, S. villosum, S. eldoretianum, S. americanum").
In Taiwan, growing cabbage (Brassica oleracea) and Chinese cabbage (Brassica rapa) with other vegetables such as "tomato, dill, garlic, safflower, oat, and barley" lowered damage to the cabbages by the pest Plutella xylostella, or diamondback moth (Talekar et al. 1988).The diamondback moth (DBM), whose larvae eat Brassica leaf tissue, is a major pest of crucifers (plants in the mustard family) worldwide. UC Farm Advisors in Ventura and Santa Maria found that intercropping does work, in "small plots" by interspersing Brassicas amongst unrelated plants, to reduce damage by the DBM (Phillips). Another interesting point is that although sprinkler irrigation has been shown to reduce DBM damage in some areas, such as Taiwan, where researchers utilized intercropping in conjunction with overhead sprinkler irrigation (Talekar et al. 1988), in the cool central coast areas of California, using sprinkler irrigation increased incidence of disease on Brassica crops (Phillips).
In Bangladesh, Brassica napus (var. Bari Saiesha-7) was grown with garlic and yields were increased, and aphid numbers were reduced (Sarker et. al. 2007).
In the UK, when Brassica cultivars were grown with (50 cm. apart or less) taxonomically unrelated plants, (for example, legumes, tomatoes, dill, lettuce) cabbage root fly (Delia brassicae) and cabbage aphid (Brevicoryne brassicae) damage was significantly reduced (Tukahirwa & Coaker, 1982).This year I will be trying kale, potatoes, garlic, Calamintha, verbena, and Senecio, growing in large pots together. What are you growing ?
Potatoes and Kale. Chop potatoes (Yukon Gold variety, with a few Purple thrown in is good) in chunks, remove skin or not. Simmer in water until soft. Shred kale very thinly; add to potatoes, and cook just a few minutes until kale is just softened. Mash all together. Add salt and butter to taste. Shredded White Cheddar is also a good topping.
Altieri, M.A. & S. Gliessman. 1983. Effects of Plant Diversity and Herbivory of the Flea Beetle, Phyllotreta cruciferae Goeze, in California collard (Brassica oleracea) Cropping Systems. Crop Protection. 2:4. 497-501.
Coaker, T.H. 1980. Insect Pest Management in Brassica Crops by Inter-Cropping. Bulletin SROP. 3. 117-125.
Oseko J.K., M.O. Abukutsa Onyango & J.C. Onyango. Performance of African Kale (Brassica carinata) to Intercropping With Other Indigenous Vegetables. IN Isutsa et al. (Eds). 2002. Proceedings of the Fifth Workshop on Sustainable Horticultural Production in the Tropics. Nov. 23-26, 2005. Egerton University, Njoro, Kenya.
Phillips, P.A. (UC Farm Advisor, Ventura, CA 93003) Diamondback Moth: A Key Pest of Cruciferous Crops. cesantabarbara.ucanr.edu/files/75293.pdf
Sarker, P.K., M.M. Rahman & B.C. Das. 2007. Effect of Intercropping Mustard With Onion & Garlic on Aphid Population & Yield. J. bio-sci. 15. 35-40.
Talekar, N.S., S.T. Lee & S.W. Huang. 1988. Intercropping and Modification of Irrigation Method for the Control of Diamondback Moth. AVRDC, Shanua, Tainan, Taiwan, ROC. eap.mcgill.ca/CPCM_3.htm
Theunissen, J., C.J.H. Booij, G. Schelling, & J. Noorlander. 1992. Intercropping White Cabbage With Clover. Bulletin OILB/SROP. 15: 4. 104-114.
Tukahirwa, E.M. & T.H. Coaker. 1982. Effect of Mixed Cropping on Some Insect Pests of Brassicas......... Entomologia Experimentalis et Applicata. 32: 129-140.