Solving the Algae Problem

Algae on the surface of the topping mix can now become a problem. You will see algae as a greenish crust on the wet soil surface between the plants. If left unchecked, it will put a girdle around the plants and suppress their growth. I try to overcome this using the simple method shown in the photo below.

The top dressing of coarse stones prevents algae from growing on the wet surface of the potting mix. In this photograph, the surface of the topping mix where the algae was starting to form, has been covered with a few mm. of coarse beach grit. This will make it too dark for the algae to survive.

Choosing the Particle Size

The particle size for this grit is crucial. If it is too fine, it will remain constantly moist by capillary action from below or by water being sprayed onto the seedlings from above. It will then grow its own fine crop of algae and nothing will have been achieved. It needs to be a size that is normally dry even when the lid is fitted, but small enough not to overpower the delicate seedlings. I use the beach grit, but if I had not discovered it, I would have been looking what was available in agricultural stores for bird grit, swimming pool stores for coarse filter sand, etc

Note: The downside to this grit topping is that you can not see how wet the growing medium is.