Companion plants constitute a form of biological control – the use of living organisms to manage unwanted pests and disease organisms. Cannabis plants have been grown as companion plants alongside crops which require this protection. Riley (1885) noted that Cannabis sativa growing near cotton exerted a “protective influence” against cotton worms (Alabama argillacea, then called Aletia xylina). Similarly, hemp grown around vegetable fields safeguarded the fields from attack by a cabbage caterpillar, Pieris brassicae (Beling 1932); potato fields were protected against the potato beetle, Leptinotarsa decemlineata (Stratii 1976); wheat suffered less damage by the root maggot, Delia coarctata (Pakhomov and Potushanskii 1977); and root exudates of Cannabis repelled underground larvae of the European chafer Melolontha melolontha (Mateeva 1995). Some of these reports have been refuted in subsequent studies (Ziarkiewicz and Anasiewicz 1961, Mackiewicz 1962, Kurilov and Kukhta 1977).

Cannabis suppresses the growth of neighboring plants, whether they are noxious chickweed, Stellaria media (Stupnicka-Rodzynkiewicz 1970) or valuable crops such as lupine, beets, brassicas (Good 1953) and maize (Pandey and Mishra 1982). Hemp has been interplanted with potatoes to deter the potato blight fungus, Phytophthora infestans (Israel 1981).

Hemp has been rotated with potatoes to suppress the potato cyst nematode, Heterodera rostochiensis (Kir’yanova and Krall 1971). Hemp rotations also suppressed soil populations of the root knot nematode, Meloidogyne chitwoodi (Kok et al. 1994). Some cultivars of Cannabis are resistant to Meloidogyne hapla (de Meijer 1993). Scheifele et al. (1997) assessed the soil populations of several nematodes, before and after a hemp crop (using cultivars ‘Unico B’ and ‘Kompolti’) in Ontario, Canada. The hemp crop suppressed soybean cyst nematodes (Heterodera glycines), but increased the populations of spiral nematodes (Heliocotylenchus or Scutellonema species) and root knot nematodes (Meloidogyne incognita).

Mateeva (1995) studied an unspecified Meloidogyne on four different crops growing in Bulgarian soil. After 30 days, cucumber plants averaged 56 root knots per plant and 396 Meloidoygne larvae were found in the surrounding soil. Tomato plants averaged 42 root knots and 318 larvae, Cannabis plants averaged 5 root knots and 21 larvae, and marigolds averaged 1 root knot and no larvae. Mateeva concluded, “by including unfriendly plants in the rotation scheme with tomato and cucumber, it is possible to obtain a soil completely cleared from root knot nematodes.”

There’s more at the URL. The report does say that the value of some of these studies have been “debated”. Nonetheless, with the federal drug laws as they are in relation to cannabis, farmers would be disallowed from using these particular crop rotations and interspersing their crops with hemp as a natural or organic biological control method, without receiving some kind of special permission from the government.

In the area that I live in, phytopthora is a large problem with some avocado orchards, and besides careful selection of both rootstocks and clay-free planting areas, various chemical methods are used to control it on existing trees if and when it strikes. The local phytopthora problem is commonly referred to as Avocado Root Rot. While potatoes are a much different type of crop, one wonders whether such springtime hemp companion planting would be helpful with avocados and their issues with phytophthora cinnamomi, or whether such companion planting would be suppressive to the avocado trees or their fruit production.