Greenhouses are common on organic farms. They are often used for season extension such as starting frost-tender seedlings, keeping greens going over the winter or providing extra heat for plants grown in beds or in the soil of the greenhouse.
Less common is the production of potted plants in a greenhouse, yet researchers from Maine, New York and Kansas have concluded that “With careful management, it is possible to grow container plants in organic production systems that are comparable to those produced in conventional production systems.”
A key part of the management is the potting mix. Most commercial potting mixes contain synthetic chemicals used as fertilizers and wetting agents. These chemicals are not allowed in organic production. Many growers who make their own potting mixes use compost to make up 20-50 per cent of the mix (by volume) along with ingredients such as peat moss, pine bark, perlite or vermiculite.
Compost alone cannot provide the fertility needed to sustain plants that are grown in pots for long periods. Nutrients can be provided by alfalfa meal, soybean meal, kelp, fish meal/powder, pelleted chicken manure, feather meal, seabird or bat guano, bone meal and blood meal. Mined sources of nutrients include limestone, glauconite (i.e., greensand), magnesium sulphate, rock phosphate and potassium sulphate (if from langbeinite or other non-synthetic sources). American organic growers often provide supplemental nutrients after four to five weeks.
Note: according to the Canadian Organic Standards, potting soil used in a container system, with the exception of transplants, “shall provide nutrients to plants continuously. The soil (growth media) shall contain a mineral fraction (sand, silt or clay) and an organic fraction; it shall support life and ecosystem diversity.”
Substrates and fertilizers for organic container production of herbs, vegetables, and herbaceous ornamental plants grown in greenhouses in the United States. Stephanie E. Burnett, Neil S. Mattson, Kimberly A. Williams. 2016. Scientia Horticulturae. Vol. 208. No. 2016. pp. 111–119; CAN/CGSB-32.310. Organic Production Systems: General Principles and Management Standards. Canadian General Standards Board. 2015
- Janet Wallace