Starting Seeds in Rockwool

Not just for hydroponic growing

It was a little over a year ago that I experimented with the Kratky hydroponic method of growing plants over winter. Using this method meant using a growing media called rockwool (Grodan) to germinate the seeds in. Much like seed starting mix, there is no nutrition within the media, but it does provide a moist environment for germination to occur and has structure to it that allows the roots to take hold of.

Rockwool/Grodan comes in different sized blocks.

I am still blown away in awe and wonder at the life that is perfectly packed into the tiniest of seeds. A seed contains all the nutrition it needs to get started in life and doesn’t require any additional nutrition to germinate. In some cases seeds will have a difficult time germinating in nutrient dense soils. Think of a seed like an egg that contains the yolk sack, which is all the nutrition a chick needs to grow and develop until it hatches and then requires water and food to continue growing. Seedlings use that stored energy in the germination process when they are awoken out of dormancy by warmth and humidity. Its after germination and development of roots that true leaves start to develop. At this point you will need to provide adequate light, nutrients and moisture to promote growth into plants that will be ready for the garden.

When I Use Rockwool Instead of Seed Starting Mix

For early plant starts I typically like to use seed starting mix, especially for very small herb seeds. However, last year I decided to cut up a few blocks of water soaked rockwool into small pieces and sow my pepper and tomato seeds directly on top of the rockwool. I place these pieces in a covered container to retain the humidity and let them sit at room temperature (no germination mat required for good results). I was impressed with the final results and decided to do it this away again this year. I used only 4 pieces of rock wool to germinate over 100 seeds.

Rockwood Advantages

One advantage is that if I plant seeds in seed starting mix, there is no guarantee that I will get germination. This way, I only plant seedlings that successfully germinated and I can plant them directly into potting soil. Another advantage is if you are using older seeds, the germination rate can decline over time. Instead of wasting soil on seeds you aren’t sure are viable, you will only use soil for the seedlings produced. If you are seed saving for the first time and have concerns that the seeds you saved may not be viable, again this is a good way to germinate seedlings without using growing space and soil for seeds that may not be any good.

Some people use a method of germination with seeds placed on damp paper towels, folding the paper towel over the seed, and then putting this into plastic baggies. This method does work. However, I like the rockwool method better because the roots affix to the rockwool, acting like an anchor, and reduce the likelihood of damaging the delicate roots of the seedlings when transplanting them.

Recycled candy box with a clear lid used to start rows of tomato and pepper seeds on saturated pieces of rockwool

Sharpie marker labeled lid cover to identify the seed varieties in the rows I created with the rockwool

Tomato seeds will always germinate faster than pepper seeds. Some pepper seeds are much slower to germinate than others. This is one reason why you typically would start peppers a few weeks before tomatoes. However, both tomatoes and peppers like the same temperatures and moisture levels for germination. You’ll just move the tomatoes out sooner. By the time my peppers were germinating, I had already transplanted my tomato seedlings into pots filled with potting soil mix.

You need to place your germination container under grow lights at the point that your seeds start to germinate so that the young seedlings get the light that they need right away. You can always move seeds that are further along into other containers if need be. I love to use sushi containers for this purpose.

Sprouting pepper seeds. The white, fuzzy legs are the roots. Visuals you don’t have when you plant seeds in the soil to germinate. I use a spray bottle with water to make sure that the rockwool and the seed coats stay moist and leave the lid on during this stage to act as a green house. Note: no grow lights are needed at this stage.

Pepper seedlings with roots firmly growing into the rockwool. It is at this stage that you want to supply light for the emerging seedlings. A few days after they emerge I transplant them into potting soil. (This is a sushi container with a clear lid reused as germinating container)

A reused 8-pack partially filled with potting mix, rockwool pepper starts placed on top of the moistened soil and then back filled with more potting mix as seen in the top right corner. This will be placed under grow lights for 16 hours a day, maintaining a temperature above 50 F. To avoid “damping off” (too much moisture causes the stem to rot at the soil level), I water when the soil on top appears dry (some do bottom watering to prevent this as well). Note: to have stronger stems on your plants, make sure you run you hands over them lightly a few times a day or have a light fan or breeze to blow on them.

I hope you will give this rockwool germinating method a try, especially if you have seed packets from many years ago. Don’t throw away your seeds or worry about wasting your time planting them in seed starting mix and give this method a try.