How to Start Lettuce Seeds in Rockwool
Here’s how I successfully started lettuce seeds in rockwool cubes, after a couple of failures because I got complacent.
Starting seeds in rockwool should be easy, right?
When I started doing hydroponics a few years back, I think I just got lucky – everything I planted germinated on the first try. Over time, I got a little sloppy. I stopped using distilled water. I didn’t worry about the room temperature. I just dropped a couple seeds into grow plugs, and it worked.
This year, when I planted lettuce seeds in rockwool, and nothing came up, I was surprised. Bad seeds? I tried new seeds; still no luck.
So, I read up on starting lettuce seeds in rockwool.
Turns out – if you follow the rockwool manufacturer’s instructions, it works better. Who knew?
In this case, I used 1” rockwool cubes from SKINNYBUNNY. They tell you on their Amazon notes that the rockwool is naturally alkaline (pH is greater than 7), and you need to soak it in acidic water until the pH is stable between 5.5 and 6.5 for 24 hours.
My tap water comes out of the faucet with a pH of around 7. I bought a gallon of distilled water from Kroger, and it tested around 6.
So here’s what I did that finally worked.
I needed 18 lettuce seedlings. I’m planting them on a raft from Beaver Plastics (Beaver Plastics Lettuce Raft — bpgrower.com), and there are 18 holes that accept the 1” cubes.
I used scissors to separate 4 rows (5 cubes in each row) from the sheet of 50. I did NOT separate the cubes individually at this point; that was just my choice.
Water
I soaked the rockwool cubes in the DISTILLED water for a while and tested the water – the pH was still very close to 6. I didn’t adjust it. If needed, to hit that 5.5 – 6.5 pH window, I would have used pH Down from General Hydroponics. I keep some on hand. (The only time I’ve ever had to use pH UP was when I had to correct the pH because I added too much DOWN.)
After soaking the cubes, I took them out of the water and let them sit for an hour or so to let the excess water drain out. After that hour, they still felt heavy. Rockwool retains a surprising amount of water, and that’s good.
Seeds
Next, I dropped a couple lettuce seeds in each hole in the top of the cubes. I used a toothpick to push the seeds down about ¼” or so. After each hole had seeds, I lightly pinched the holes closed with my fingers.
Note – the toothpick in the picture below is just to remind me later which seeds are on the right (toothpick) side.
Light
I’ve started lettuce under lights before, but this time I just put the rockwool cubes in my tray, put the transparent cover over it to keep them from drying out, and sat the whole thing on the kitchen table. No light other than ambient light from a nearby window. After about 3 days, we had seedlings popping up.
Once leaves were showing, I started turning on the single-bulb fluorescent lamp and leaving it on for about 12 hours per day. That’s when the leaves started turning green.
From my experiences, light or dark is OK for germination.
Temperature
Can it be too hot? Too cold? Yes.
Most folks say lettuce germinates best between 60 and 85°F (16 – 30 C.)
Today it’s 110°F (43 C) where we live in Texas. We keep it around 76°F (24 C) in the house. The garage right now is ~ 94°F in the daytime; that’s where the plants are now, but that’s another reason I started the seeds in the kitchen.
Summary:
Before adding the seeds:
Water – soak the rockwool in distilled water and get the pH between 5.5 and 6.5.
Add the seeds and put the rockwool somewhere stable, with:
Light or dark – doesn’t really matter until the plants have germinated.
Temperature – 60 and 85°F (16 – 30 C.)
You’ll have small plants in a couple days.
Use that time to prep the area where you plan to transplant them. If you don’t move them when they’re ready, they’ll get too “leggy.” That means they’ll be long and spindly.
In the garage I have a home-made wooden frame, lined with plastic sheeting, and filled with approximately 42 gallons of water. The Beaver Plastics raft floats on top. I have 2 air stones that bubble 24/7. I use General Hydroponics nutrients at about half the recommended strengths. I use T5 fluorescent lights, on a timer, so the lights are on about 14 hours per day. I’d get better results with other lights, but this is where I am at the moment.
Before moving the new plants in, I:
- Filled the reservoir with water from the hose – I’m not buying 42+ gallons of distilled water,
- Added my nutrients, and
- Spent a couple days adjusting the pH to just under 6. It takes a while to settle and it changes when you add nutrients. I aimed for a pH of 5.8, but I’m using a General Hydroponics pH Test Kit and I can’t determine it to that level of accuracy.
- That garage temp is still high, but the plants are looking good. The pic below is Day 18.
My wife thought the plants were lonely, so she put a few figurines out there.
We’ll be eating fresh salad in a few weeks!