“I bought 8qt of Black Gold Cactus Mix and started repotting my plants. The first bag looked fine, but the second one was covered in orange/rusty colored powder and some fuzzy white blooms. I haven’t used it, but will this happen to the other one as well? What should I do with it?” Question from Rusty of St Louis, Missouri
Answer: It may look funky, but there is no need to worry. The reddish-brown growth found on the top of the potting mix is a harmless saprophytic slime mold. These exist in healthy natural environments and obtain food from dead or decaying organic matter, such as peat moss, rotting stumps, and decaying leaves (also compost). They are rarely able to grow in bags of mix, but clearly, there was enough air, moisture, and warmth in the plastic packaging to allow this misplaced saprophyte to grow and feed on the peat (or decaying organic matter). The saprophytic mold will not harm your cacti, succulents, or other plants in any way. In fact, once the bag is opened, the saprophyte will dry up, dissipate, and never return.
I hope that this helps!
Black Gold Horticulturist