Rock Dust Helps Indoor Soil Stay Free of Fungus Gnats


All-natural rock dust delivers a multitude of benefits, chief among them is helping plants boost their resistance to disease and predators. But not all rock dust is alike. Ground volcanic basalt, which is high in silicon, is particularly effective at helping plants defend themselves.

The Benefits of Silicon (Si)

A key component of cell walls, silicon helps improve plant structure which in turn increases their resistance to a variety of environmental stressors. Plants that don’t have access to silicon in the soil are stressed out and weak, making them less able to resist injuries causes by insects and pests. (Learn more in a previous blog post, “How rock dust boosts plants’ resistance to aphids and other pesky pests.”)

Controlling Indoor Fungus Gnats

Most gardeners are familiar with fungus gnats, a common indoor nuisance and in greenhouses where humidity and moisture are high. As adults, flying fungus gnats are annoying but harmless. It’s the hungry larvae that feed in the soil that can cause damage to tender plant roots. Symptoms include poor plant growth, yellowing, and wilting. Whether you are a commercial greenhouse grower or an indoor hobbyist, these small gnats can be a big problem.

Case Study: Pierce County (WA) Master Gardeners

Last year the Pierce County Master Gardeners propagated over 1,200 tomato plants and 2,000 perennials to study and to donate for the group’s annual spring plant sale. This year, they are using all-natural Cascade Minerals Remineralizing Booster, made of 100% volcanic basalt, as a soil topper on all plants started from cuttings. About 60 Master Gardeners are participating in the mineral study, and the results so far have been positive:

The Pierce County Master Gardener propagation group is having great success using the ground basalt as a soil dressing on all of our plant cuttings. Everyone is delighted with the ground basalt, as the plants are thriving. We’ve also found that the soil has stayed free of fungus gnats. Our members are excited about using it for their own greenhouses! – Cynthia LaFleur, WSU Pierce County Master Gardeners

For more information about the WSU Pierce County Master Gardeners program including upcoming plant sales and other events, please visit