Algae requires light to survive and when there is none, it dies. UVA-inhibited Sunleaves Block Covers protect your garden from algae growth by preventing the light from penetrating down to the growing medium. 

Sunleaves gives you 40 Block Covers per package so you'll have plenty to go around. These versatile rockwool / stonewool covers are white on one side, and black on the other.

When the white side is placed up, the Block Covers reflect the light back up to the leaves, giving your plants increased lighting.

If you're germinating seeds or growing heat-loving plants, such as tomatoes, place the Sunleaves Block Cover with the black side up so it will absorb the light's heat. This will increase the temperature of the growing medium and plant roots to encourage strong healthy plant growth!