The fungus that causes root rot thrives in soil that is water-logged and over-watered. The best way to protect your plants is to plant them in well-drained soil. However, not every garden offers that option. Fortunately, root rot can be prevented even in poorly-drained soils by using expanded clay. Also known as fired clay pebbles or grow rocks, this porous soil amendment maximizes drainage to prevent the growth of root rot.
Dig your planting hole(s) 4 inches deeper than they need to be to amend them with expanded clay. For example, if you are planting container plants, dig the hole a little wider than the container, but 4 inches deeper. Line the bottom of the each hole with 3 inches of expanded clay. Cover the expanded clay with 1 inch of the excavated soil.
Till large planting areas to a depth of 4 inches with a rototiller or hand tiller to amend with expanded clay. Spread 5 inches of expanded clay onto the top of the soil. Till the soil again to a depth of 6 inches.
Plant as usual and water the soil until it is thoroughly moist.