Sterilizing
Marijuana Horticulture
by Jorge Cervantes
To reuse a growing medium, it must be sterilized to remove destructive pests and diseases. Sterilizing is less expensive and often easier than replacing the growing medium. Sterilizing works best on rigid growing mediums that do not lose their shape such as gravel, expanded clay, and mica. Avoid sterilizing and reusing substrates that compact and lose structure such as rockwool, coconut coir, peat moss, perlite, and vermiculite. Avoid problems caused by compaction and dead roots by replacing used growing mediums. Once sterilized, the medium is free of harmful microorganisms including bacteria and fungi, plus pests and their eggs.
Remove roots from the growing medium before sterilizing. A three to four month old marijuana plant has a root mass about the size of an old desk telephone. Separate the medium by shaking and pulling roots away. Bounce the medium on a screen so roots come to the top. Scoop up and remove roots by hand. Fewer decaying roots cause fewer pest and disease problems, and decrease incidence of clogged feeder tubes.
Substrate can also be washed in a large container such as a barrel or bathtub. Washing works best with lighter substrates such as expanded clay or mica. Roots float to the top and are readily skimmed off with a screen or by hand.
Once roots are removed, soak the substrate in a sterilant such as a five percent laundry bleach (calcium or sodium hypochlorite) solution for at least an hour. Or mix hydrochloric acid, the kind used in hot tubs and swimming pools. Pour, drain, or pump off the sterilant, and flush the medium with plenty of fresh water. A bathtub and a shower nozzle on a hose are perfect for washing substrate. Place the substrate in the bathtub, set a screen over the drain, and use the shower head or a hose to wash down the medium. It may be necessary to fill the tub with fresh water and drain it a couple of times to rinse any residual sterilants from the substrate.
If you decide to use rockwool or coco a second time, you may have problems with pests and diseases. In general, I recommend reusing a medium only if it does not deteriorate or compact. Examples include: pea gravel, expanded clay, lava rock, sand, etc. Once used indoors, reuse rockwool and biodegradable coco in the outdoor garden.
To sterilize a hydroponic garden, remove the nutrient solution from the reservoir. Pump the solution into the outdoor garden. Avoid pumping it down household drains, and definitely do not pump it int a septic tank. The nutrients will disrupt the chemistry!
Flood the growing medium with the sterilizing solution for at least one-half hour, let drain, and flush again. Pump the bleach solution out of the system and down the drain. Do not dump the sterilants outdoors; they will defoliate plants where they are dumped. Use lots of fresh water to leach and flush the entire system including beds, connecting hoses, drains, and reservoir. Make sure all residues are gone by flushing entire system twice for a half hour. Remove all solution from the tank, and scrub rockwool outside on a sheet of black plastic, and cover with black plastic. Let the sun bake the layer of slabs, or flock, for several days. The temperatures in the rockwool will climb to 140F or more, enough to sterilize for most all harmful diseases and pests.