I soaked my rockwool cubes and planted basil seeds 5 days ago. Only 2 out of 10 have sprouted. Are the cubes too wet? Should I be using a dome? Temperature is 68°F. What's the ideal germination setup?

Your 20% germination rate at day 5 is telling you something specific: the conditions aren't wrong enough to kill seeds entirely, but they're suboptimal enough that germination is stalled. Here's the diagnosis.

Temperature is the primary culprit

Basil is tropical in origin and wants warmth to germinate reliably. The research is clear:

• 77–80°F (25–27°C): 90–95% germination in 3–5 days
• 72–75°F: 70–85% germination in 5–7 days
• 68°F: 40–60% germination, taking 7–10+ days for those that do sprout
• Below 65°F: Germination drops below 30% and becomes unreliable

At 68°F, your seeds aren't dead - they're just germinating slowly. Most will eventually sprout, but the delay and low rate are temperature-driven.

Fix: Use a seedling heat mat under your rockwool tray to raise the medium temperature to 75–78°F. An inexpensive heat mat ($15–$25) dramatically improves germination across almost all culinary herbs. Check the temperature with a probe thermometer directly in the rockwool - ambient air temperature can read 68°F while the mat brings the medium itself to 76°F.

Rockwool moisture: too wet is a real problem

Rockwool should be wrung out firmly after soaking - squeeze until water stops dripping freely, so the cube is moist but not saturated. Over-wet rockwool creates anaerobic conditions around the seed that can inhibit germination and encourage damping-off (seedling fungal collapse).

Correct prep:

1. Soak cubes in pH 5.5 water for 30–60 minutes (rockwool is alkaline out of the bag; acidifying is important)
2. Remove and shake firmly, then squeeze once - the cube should feel like a wrung-out sponge
3. Place seeds in the pre-formed hole at 1/4 inch depth; do not bury deeply
4. Mist the surface of the cube lightly once daily; do not re-soak

Should you use a humidity dome?

Yes - particularly without a dome, surface moisture evaporates and the top layer of rockwool where your seeds sit dries out between mistings. A simple clear plastic dome (or even plastic wrap over the tray) maintains 80–90% humidity and keeps seeds from desiccating. Remove the dome once seedlings emerge and show their first true leaves.

Should seeds be in the dark?

Most herb seeds (including basil) are not light-dependent for germination - light neither helps nor hurts. Darkness is fine until sprouts emerge; once you see seedlings, move them under your grow light immediately to prevent etiolation (stretching toward light).

What to do now

Your 2 sprouted seedlings are fine - move them under light. For the remaining 8 cubes:

1. Add a heat mat to raise medium to 75–78°F
2. Add a humidity dome
3. Wait another 5–7 days before declaring them failed

At improved temperature, most of those 8 should still germinate. Basil seeds remain viable at room temperature for 3–5 years, so the seeds themselves are not likely the issue.

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This answer is reviewed against published horticultural research. It is not a substitute for advice from a certified horticulturalist.