Cultivation in New Zealand
Commercial mushroom cultivation in New Zealand began in the 1930s when the Brightwell brothers of Avondale, Auckland, imported button mushroom (Agaricus bisporus) spawn from Britain and grew them in a heated shed. By today’s standards their methods were fairly haphazard.
In the 1960s, cultivating and sterilising techniques had been refined enough to provide reliable yields, and a mushroom industry began to develop. Until then, most New Zealanders could only eat mushrooms for a brief period each year, when field and horse mushrooms (Agaricus campestris and A. arvensis) appeared in paddocks after the first autumn rains.
Annual production in 2007 was about 7,500 tonnes, most of which was consumed fresh within New Zealand. A small amount was canned, and some was exported fresh to Asia and Australia. The industry was dominated by two large growers, who supplied the major supermarkets. Compared to the productivity levels (weight of crop produced per area) of European and North American growers, New Zealand’s productivity was low.
There are three main stages of mushroom cultivation:
- preparing mushroom spawn
- preparing a growing medium (compost)
- growing mushrooms on compost.
These stages are quite separate. They require different facilities, and are often done by different companies.
A small piece of mushroom is grown in tissue culture, where it spreads by fine filaments called mycelia. Sterilised cereal-grain seeds such as rye, wheat or millet are inoculated with pieces of mycelia. This spawn is then used to introduce the mushroom fungus to the compost on which it will grow.
Button mushrooms need a food source, which must be decomposed. In New Zealand, mushroom farmers make their own compost from a mix of moist wheat or barley straw, chicken and/or horse manure, and gypsum (calcium sulfate).
The mix is allowed to compost for three weeks, during which time it is regularly turned to aerate it.
The different mushrooms on sale – white buttons, Swiss browns or crimini, and Portobello flats – are different strains of the same species. In nature, Agaricus bisporus mushrooms have brown caps – the white-capped variety is a mutant type that was selected for cultivation.
When the compost is ready, it is loaded into growing containers (wooden trays, growing shelves or plastic bags) and pasteurised at 60°C to kill any pests. Mushroom spawn is then mixed through and left for two to three weeks to grow and completely colonise the compost. A damp casing of peat, lime and bentonite is then placed over the mixture, and helps stop it drying out. The temperature, humidity and carbon dioxide levels in the growing rooms are adjusted to stimulate the spawn to form reproductive structures (button mushrooms).
Mushrooms first appear on the compost surface 15–21 days later. They grow in a series of flushes, about three to five days apart. Two to four flushes are hand-harvested from each container. By picking mushrooms at different stages of maturity, growers get buttons with unopened caps, cups with open caps showing the gills underneath, and flats with large expanded caps and showy gills.