Is it safe to grow things you plan to eat alongside flowers/trees and such if everything is on the same reservoir? Can some plants’ roots put out things that might be toxic and also be taken up by, say, a tomato plant?

This is an interesting question as recirculating systems share the same nutrient solution, so any chemicals or compounds released from the roots could potentially influence the growth of all species in that system.

Plant roots are known to produce exudates, sometimes in large quantities. These consist of organic compounds such as amino acids, organic acids, sugars, a wide range of carbohydrates, phenolics, lignins, fatty acids, sterols, enzymes, mucilage and proteins, as well as ions and inorganic acids. Even the most commonly grown hydroponic plants produce these root exudates and generally they are not an issue as most are broken down relatively quickly by beneficial microbes which are present in the environment and nutrient solution.

The problem that can occur with certain species, such as eucalyptus, is termed “allelopathy,” where plants produce compounds that directly inhibit the growth of other surrounding plants. These allelopathic chemicals are often contained in leaves, roots, fruit, flowers, nuts, or stems. In many instances it is the leaf litter dropped by the allelopathy species which acts to suppress the germination and seedling growth of other species in the immediate vicinity. This gives the allelopathic plant a competitive advantage for light, nutrients, water, and space, and helps ensure its survival.

However, roots of some species can also produce allelopathic chemicals, so the potential for these to be released into a hydroponic nutrient solution does exist.

Tomatoes are a crop that is particularly sensitive to allelopathic compounds released by some species. The food crops commonly grown in recirculating hydroponic systems are generally not allelopathic so can be cropped together with no real risk. However, avoiding plants that are known to produce allelopathic compounds would be advisable — more because these inhibitory compounds could severely restrict growth, or at least slow development, of the other fruit/vegetable species in a mixed system rather then becoming toxic to eat.

Eucalyptus would be best grown in its own substrate-based system or in pots. While small seedlings don’t pose much risk as the amount of allopathic compounds they produce would be fairly minimal, large specimens could create issues in a mixed system.

There are only a few plants species that have known allelopathic effects. These include tree, crop, and weed species such as barley, oats, wheat, rye, canola, mustard, buckwheat, red clover, white clover, sweet cover, hairy vetch, creeping red fescue, tall fescue, perennial ryegrass, black walnut, fragrant sumac, rice, pea, sorghum, eucalyptus, and a few others.

The inhibition of growth in a mixed system containing these types of species would, of course, depend on the size and number of the allelopathic species, the volume of recirculating nutrient solution and how often that solution was replaced, the breakdown of toxic root exudates compounds by microbes in the nutrient solution, and use of other treatments such as ozone, or activated carbon filtration. Generally, however, allelopathic species are not widely grown hydroponic crops and if they are to be produced, it would be best to isolate these in a separate system.

– Lynette Morgan
Suntec International Hydroponic Consultants