At least once a year, preferably during the summer months when the plants can be moved outside, give the greenhouse a thorough cleaning. You can sterilize all the benches, ground, interior plastic, any pots, etc. with a solution of household bleach mixed 1 to 10. Remove any unused pots, containers, trash, etc, so no little guests will have a home over the winter. An aphid can live under a leaf in a greenhouse to survive until Spring comes and hatch a major population at that time. Spider mites are also great at hiding, especially in sand floors. Sterilize all your tools, scissors, knives, potting bins and anything else you use on your plants or to dig in the dirt.