Soil Molds and Fungus


Some kinds of mold and fungus can be problematic for beginning gardeners so we should consider this before the season starts. Many of the potting soils and mixes that you get at your local big box stores are contaminated with fungal spores and gnats, bringing these into your grow room will cause problems from day one.

A good idea would be to pay a few extra dollars up front and buy (or make your own potting soil) inert growing mediums that do not contain organic constituents or time release fertilizers. Using inert mediums for seedlings and cuttings is nice because there is nothing for fungus to feed on. Pro-Mix HP or Sunshine mix are great 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, perlite, & lime. Don’t apply nutrients or anything with biological activity until after the seedlings grow roots. Once plants show their first true set of leaves (2nd node) they will need to be fertilized.

Pro Mix Inert Growing Mediums

If you are set on using biologically active mediums and composts or are already invested in this direction, you can attempt to control harmful soil fungus’ by using one of the following methods:

  1. Calcitic Lime: Lime is a great fungicide in the soil and has the added benefit of providing pH buffering and Calcium for plants. Mix 1 tsp of powdered lime into a gallon of water and water as usual. Be careful when applying lime via solution as too much will adjust the mediums pH to a solid 7.0 causing nutrient lockout.
  2. 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. This works well and can also be used on leaf surfaces of established plants for controlling things like powdery mildew.
  3. 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. Neem is great as an insecticide against spider mites.
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