In organic and biodynamic farming, plant-based preparations are used in place of chemical pesticides — safe for people and the environment. One of the most widely used preparations is nettle.
Some wild potato species, in order to defend themselves against attack by aphids, secrete a volatile substance very similar to the one aphids release in the presence of danger, thereby causing the parasites to disperse. Solanine itself, an alkaloid found in abundance in all solanaceous plants (potatoes, tomatoes, tobacco, aubergines, peppers), represents a form of plant defence against attack by animal parasites (Colorado beetle) and fungal ones (fusarium). These observations gave rise, as early as the beginning of the century, to the idea of using plant-based preparations to protect plants — a practice now rediscovered by organic farming.
One of the plants most widely used for this purpose is nettle. The high content of salicylic acid and formic acid makes nettle-based preparations an effective means of combating the spread of numerous animal and plant parasites. All nettle-based formulations are prepared using the whole plant, without roots, in a quantity of one kilogram of fresh plant or 200 grams of dried plant per 10 litres of water.
Nettle macerate is prepared by leaving nettle to soak in cold water until the liquid becomes very dark and no longer produces any foam. To speed up the process, the container can be placed in the sun. A handful of lithothamn (seaweed flour) or a few leaves of angelica can be useful for neutralising the strong odour given off by the preparation during fermentation. The macerate should be stirred every day, and is ready after approximately 7–10 days.
Undiluted nettle macerate, applied directly to the heap, is useful for accelerating the maturation of compost, while sprayed onto the soil it keeps ants away. Diluted with water at a ratio of 1:20, it is effective for strengthening and accelerating the growth of young seedlings. At the same dilution, applied for three consecutive days at two-week intervals, it is useful against cryptogamic diseases (various rots, peach leaf curl, downy mildew, scab), numerous insects (moths, cherry fly, pea weevil) and mites (red spider mite). The macerate can also be used to soak the roots of seedlings before planting, or to disinfect seeds. At a dilution of one tenth, the macerate is an effective remedy for reviving wilted plants by bathing their roots, immersing the pot in the solution.
A particular nettle-based preparation is the "fermenting macerate", obtained through incomplete maceration of just 12–24 hours. It is used diluted with water at a ratio of one to 50 against aphids, scale insects and mites, applying three treatments spaced three days apart. Nettle macerate should never be used on cabbages and all other brassicas, as its strong odour attracts the cabbage white butterfly, nor on vegetables and fruit intended for storage, in particular pumpkins and apples; while treatments carried out on tomatoes and cucumbers must be limited exclusively to the soil in order to avoid the onset of infections.