A Guide To Mushroom Growing

by David Thornber, Growing Manager

The materials needed to cultivate mushrooms are compost, casing soil (peat) and mushroom spawn.

The main raw materials for compost preparation are straw and water.  Straw has a number of functions, it is the main source of carbon in the compost, which forms the texture of the compost heaps, allowing air to flow through it and also determines the water-holding capacity.

Manure or other nitrogen containing substances are added to this base material as well as gypsum,.  These raw materials have to be pasturised to be able to grow mushrooms. The object of pasteurisation is to destroy undesirable organisms in the compost, this is done with temperature increase for an allocated time period. 

After the compost is pasturised it is conditioned to removed ammonia and then cooled down so that the spawn can be added.  The spawn must be mixed carefully with the compost for even growth.

Compost fully grown with mushroom mycelium produces hardly any mushrooms, it has been found from experience that a layer of casing soil must be placed over the compost in order to obtain mushrooms in quantity.    The functions of casing soil are to supply water for growth and development of mycelium, and mushrooms and to form an environment that the crop enjoys to grow in.  Peat is ideal because of it’s great water holding capacity, it’s acidic nature is neutralised using sugar beet lime.

To obtain an even thickness of peat, the underlying compost must be evenly and firmly pressed into the bed.

Once the casing has been raked it is a depth of 1¾ - 2 inches.  The optimum compost temperature for growing the mycelium through the compost and into the casing layer is 25ºC.  The Mycelium must be allowed to grow to the surface of the peat, this is controlled by the application of water (more water less growth, less water more growth).

Once the growth has reached the surface of the peat ventilation may be started.

Venting is intended to slow the growth of the mycelium and make it form tiny mushrooms (pins), this is done by changing the climate of the shed or area where the mushrooms are to be grown.

Relative humidity is lowered CO²  concentration also by introducing outside air conditions and finally air temperature dropped which, therefore, lowers the compost temperature.

This climate change “shocks” the mycelium into thinking it may start to die so it produces it’s reproductive fruiting bodies which are the mushrooms we desire.

Once the “pins” have formed they are grown in stable conditions until ready for picking, this is on average involving an air temperature of 18ºC, RH 90 – 92 CO² levels of 1700 parts per million.  The crop mushrooms are very delicate and any changes in climate when cropping must be very subtle and gentle.

Once the first flush of mushrooms has be picked off, water must be added to the peat casing again so a second flush can be grown.  Because a mushroom is 97% water the peat must

be kept in a moist condition so the crop can get a supply of water throughout the crop duration.

Some farms grow a 3rd flush, after which the compost is “cooked out” which involves heating the compost up to 65ºC for 10-12 hours which kills off any bacteria and reduces contamination problems keeping a growing site clean and healthy.

The spent compost, as it is known, is very popular in horticulture for improving soil.  Once this compost has been removed, the growing area is sterilised and the process can start again with more fresh compost and casing.