Algae on rockwool

I have a friend who has a small indoor operation going on. It works well and he has a good strain, but green algae is growing all over the rockwool and entirely covers its surface. So far it doesn’t seem to have affected the plants. Do I have a problem? How can I get rid of this algae?

Personal to Flexi: Your letter truly intrigued me. You wrote me anonymously, yet you couldn’t quite admit consciously to me or perhaps to yourself that you have a garden, so you started your letter with “I have a friend.” Only later, when you got into the writing stream did you start using “I.” You called your garden an “operation.”

I was disturbed by this because this attitude places you in personal jeopardy. First, if you can’t admit to a stranger who gets lots of correspondence that you have a small garden, there is a good chance that you can’t admit it to yourself. If you were growing a plant that didn’t have legal repercussions connected to its cultivation, like for instance, tomatoes, that wouldn’t be a problem. However with the illegal status of marijuana you must be conscious of your actions and take responsibility for them yourself so that you can protect yourself.

Secondly, you should be proud of your garden. I am not implying that you should tell anyone about it, but you should relish your accomplishment and gain confidence from your success.

If your observations are that the algae is not affecting the plants there is no need to get rid of it. However, some gardeners and most hydroponic enthusiasts think it can be a problem.

One way to get rid of algae is to deprive it of light, which it needs to photosynthesize. The method is very safe because it involves no chemicals. You can affix black polyethylene plastic collars around the plant stems that cover the tops of the rockwool cubes. Other materials that can be used are white cardboard or styrofoam. If collars are inconvenient, then strips can be placed over the line of cubes to cover them.

Hydrogen peroxide can be added to the water and will deter growth of algae, but it may also affect fertilizer chemistry.

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 new book, Best of Ask Ed: Your Marijuana Questions Answered. Sorry, Ed cannot send personal replies to your questions.