Indoor Farming Tips: Rockwool

Much of the content about hydroponics online is either specific to cannabis growing, or created by enthusiastic amateurs in their spare time. So some of the guidance is confusing or unintentionally misleading. In our Intro to Indoor Farming Classes we try to cut through the fog and give simple, direct advice. 

As a specific example, over on r/hydro reddit user rabidsoggymoose (!) had a lot of questions about how to use rockwool cubes (a type of growing medium):

This is my first time using rock wool starter cubes and there seems to be some conflicting information online. I'm planting some Chinese greens like bok choy, gai lan, etc. The seeds are very, very tiny. Everyone seems to be doing things differently online. From a traditional soil-based perspective, it doesn't seem like seeds are that fussy about how they sprout, are they?

We go through his questions one by one with some simple answers.

Should I soak the cubes in just water or water + nutrient solution?

You can soak in plain water. The soaking is to balance the pH and reduce dust, not for the purpose of nutrient dosing.

Is it ok to soak the cubes in the same nutrient solution that my fully-grown plants are using?

Yes but see above. Seeds do not need nutrients to sprout. They don't really need nutrients until their cotyledons are already out and they are starting to grow their first true leaves. So in a dedicated nursery/germinator system, you might use nutrients if you don't want to have to transplant while the plants get their initial true leaves. Having nutrients has no negative effect, except obviously you have to monitor the levels and you increase likelihood of algae. Normally you might keep the nutrient levels in a nursery system at a lower EC level than a mature system.

Should I put multiple seeds or just one seed in each cube? What happens if multiple seeds sprout?

Depends on the crop's germination rate, how expensive seeds are, whether multiple plants growing per site is a problem or not... But really - to start with, and this is just one way, do 1 seed per site. Then you can see the germination rate, and next time you can add more if you need & want to 'guarantee' one germination per site with the potential for multiple plants per site (which can also be thinned out later). Main thing is to remember how many seeds you planted so you can iterate next time.

Should I allow them to sprout in sunlight or without sunlight?

Sunlight (or an artificial substitute like LED light) is not necessary for the initial sprout, but very quickly becomes necessary as the seedling grows. If kept in the dark too long, your seedlings will stretch out and this will affect the future shape of the plant negatively. So if you are not sure how long it will take for your plants to sprout, or won't be checking them every day, safer to sprout them in the light. As you get more experienced, you may experiment with that "dark" time to affect the shape / color of your plant as you wish.

Do I even need to put them in a nursery? Can't I just soak the rock wool, chuck some seeds in, and transplant them directly into my Ebb and Flow system (making sure the water flow can make direct contact with the rock wool or just dangle a piece of paper towel underneath it so capillary action will keep the rock wool wet) and let them germinate and grow there?

You can totally do that, as long as you have the correct flood height. One reason people use a 'nursery' or germination area is that while the seeds are sprouting you can keep the cubes (or whatever other growth medium you are using) spaced tightly together, using less space. In your 'mature growing' system you normally are spacing out your plants more widely - so if you germinate in that system you are being less efficient in your use of space. For home growers, not a big deal.

For a complete introduction to Indoor Farming, check out our hydroponics classes. No experience necessary. 


June 11, 2017 by David Goldstein
Tags: class learning