By the time you notice the tiny dark gray or black gnats buzzing round, one generation of these common plant pests has already infested the soil of your herb garden.
To find a natural cure for soil gnats in herb gardens you should understand the life cycle of the fungus (soil) gnat or Sciaridae.
The eight-inch long gnat you see is the mature adult and it is in the process of laying approximately 300 eggs on the moist soil surface of your garden for a new generation.
These gnats prefer to lay their eggs on damp soil which is rich in organic matter. During the winter, when overwatering is more common, the soil gnats may become a problem.
The entire life cycle of this garden pest is about four weeks in duration from egg to mature adult. The mature fungus gnat will not harm the herbs in your garden, but the larvae will. From four to six days after the eggs are laid, they hatch.
The fungus gnat larvae are white and about a quarter-inch in length with a black head.
Fungus gnat larvae burrow into the moist soil and feed upon the root system of the herb plants. Adult soil gnats live for only about ten days.
One natural cure for soil gnats in herb gardens is to first rid the soil of the infestation of eggs and larvae. Horticulturists agree that you should allow the soil to completely dry out between waterings.
Dry soil will cause the larvae to dry up. The soil should be dry for up to three inches in depth. Water only until the water comes out through the drainage hole in the bottom of the planter.
Remember that seedlings require smaller amounts of water than mature herb plants and may require watering only once a week during the winter months.
For severe infestations, the herb garden plants may need to be repotted in new soil. The old soil should be discarded. Another natural cure for soil gnats in herb gardens is to fool the gnats into believing the soil is not moist.
Do this by applying about a quarter-inch of sand to the soil surface and then covering the sand with cedar chips. The sand will dry quickly and cedar is a natural insect repellent.
Trap the mature gnats by hanging a sheet of yellow paper that has been coated with petroleum jelly close by the affected herb plant. The yellow paper attracts insects and the petroleum jelly makes them stick to the paper. This coated paper operates much like the flypaper strips you can purchase in hardware stores.
Another method is to place vegetable oil and cider vinegar in a plastic container or jar with the lid perforated with holes. The gnats will find a way to get to the cider vinegar and be trapped by the vegetable oil which floats on the surface.
Even if the gnat finds a way to get out of the jar, it will not be able to reproduce. You can prevent soil gnat infestations in your herb garden by these methods.