Molds and fungus can be problematic for beginning gardeners so we should sort that out first. Many of the potting soils and mixes that you get at your local Home Depot or Lowes will be contaminated with fungal spores and gnats, so bringing these into your grow room will cause you problems before you even get started.
The best idea is to pay a few extra dollars up front and buy inert growing mediums that do not contain organic constituents or time release fertilizers. Using inert mediums for seedlings and cuttings will starve fungus to death. Pro-Mix HP or Sunshine mix are my favorite growing mediums, these have nothing in them that fungus can feed off of. If these products are not available to you, use a mix of milled sphagnum peat, vermiculite and perlite, these are inert and will not grow mold. Nutrients are your enemy when growing from seeds or cuttings. Don’t apply any nutrients or anything with biological activity until after the seedlings grow roots. At this point they need food, before rooting they do not.
On the other hand if you are set on using biologically active mediums and composts or are already invested in this direction, you can attempt to control soil fungus by using one of two methods:
- Baking Soda: Mix 1 tablespoon of baking soda per gallon of water and put into a spray bottle. When starting from seed spray the soil surface until evenly moist and reapply as needed. I have found this to work well and can also be used on leaf surfaces of established plants for controlling things like powdery mildew.
- Neem Oil: You can purchase a Neem Oil concentrate from your local Home Depot or Lowes that acts as a fungicide. I have had mixed results when used on soil surfaces and tend to use it only on foliage should a problem pop up like powdery mildew. It also works great as an insecticide against spider mites.