I attended a garden lecture where the speaker recommended using wine corks in the garden, either ground up as a mulch, or whole in containers. She said cork would help with moisture retention in the soil. Do you agree?
The primary source of wine corks is the cork oak tree, Quercus suber. The species name is a clue to the fact that cork is largely made of suberin, a waxy hydrophobic (water-repelling) substance found in other woody plants.
It does not make sense to use a hydrophobic substance as a mulch, since mulch is meant to allow moisture to reach the roots of your plants, not to repel water. For the same reason, it does not make sense to add cork to your containers, either. Professor Linda Chalker-Scott of Washington State University says that cork will not in any way help the soil retain water.
Chalker-Scott has long advocated using arborists’ wood chips as a mulch rather than bark mulch, for similar reasons:
“Bark does not function like wood chips in its water holding capacity. Bark is the outer covering of the tree and is heavily suberized to prevent water loss. Suberin is a waxy substance that will repel water, and in fact helps explain why fresh bark mulch always seems dry. Wood chips, on the other hand, consist primarily of the inner wood, which is not suberized and has the capacity to absorb and hold moisture.”