Use larger rockwool

I’m growing in four-inch rockwool and the roots are growing heavily out the bottom. The plants are a foot tall and growing well. I want to wait for another month to start flowering. Should I grow in six-inch or will it be all right to stay in the four-inch cubes?
Cocoa, Florida

The plants will be completely rootbound in the cubes and the roots will have nothing to grow into. The solution is to place the four-inch cubes onto a rockwool slab. The roots will quickly grow into the new rockwool, securing the plant tightly so that it won’t fall over.

The added root space allows the roots to grow a larger infrastructure that is capable of supplying the upper plant with more resources. This results in more growth faster and larger buds at harvest.

If left in the four-inch cubes, the roots will struggle to supply the large plants with water and nutrients. The plants will also be top-heavy and will lean in one direction or another, blocking each other’s light.

Readers with grow questions (or answers) should send them to Ed at: Ask Ed, PMB 147, 530 Divisadero St., San Francisco, California 94117, USA. You can also email Ed at [email protected], and send queries via his websites at All featured questions will be rewarded with a copy of Ed’s book, The Big Book of Buds. Sorry, Ed cannot send personal replies to your questions.