Growing Onion & Brassica Seedlings: Trimming and Temperature Tips
📌 Quick Summary: Indoor Onion & Cabbage Seedlings
- 🧅 Should you separate onion seedlings? No! If you lack space, let them grow tightly crowded in a single container. When transplanting outside, you can plant them in “multi-sown clumps” of 3-4 onions per hole.
- ✂️ The “Combing” Technique: If your indoor onion seedlings fall over, gently comb them upright, top-dress the base with 1/4 inch (0.5 cm) of dry soil, and trim any dry tips with scissors.
- 🌡️ The Light/Temp Balance: The warmer the room, the more intense grow lights your onions need. If it’s a cloudy day with low light, you MUST lower the room temperature to around 64°F (18°C) to prevent them from getting leggy.
- 🥦 Brassica Timings: Start early broccoli and cabbage indoors in mid-March, but move them to an unheated greenhouse within a week of sprouting. They hate indoor heat!
When to Start Broccoli & Onion Seedlings
Irina, for an early harvest, I sow about 5-6 broccoli plants, which is enough to eat fresh and freeze for winter. I sow them on March 17th alongside 10 early cabbage seeds. I don’t sow them in dirt right away; I sprout them using the damp paper towel roll method.
Once the cotyledons (seed leaves) fully open, I transplant them into plug trays. But here is the catch: you cannot keep cabbage seedlings indoors after potting them up! You must take them out to an unheated greenhouse during the day. For me, that happens around March 25th (even if there is still snow on the ground outside).
As for the onions: if you have plenty of space and containers, you can separate them and pot them up individually, or in small clumps of 3-4. I don’t have extra space, so however they sprout is how they stay until planting day. In April, I just move the whole intact root ball from the cup into a greenhouse bed without dividing it.
Trimming and “Combing” Floppy Onion Seedlings
It is time for them to start putting on growth. You just need to remember the golden rule: the higher the room temperature, the more intense light they need. If you don’t have enough light right now because it’s cloudy outside, you must keep the temperature to an absolute minimum—around 64°F (18°C), no higher.
Right now, you need to “comb” your onions. Carefully lift up the ones that have flopped over and top-dress the base with a 1/4 inch (0.5 cm) layer of dry soil to support them. Use scissors to snip off any dry, withered tips.
Constantly monitor the moisture—onion seedlings should never dry out completely. Mist them from above with a fine spray if the indoor air is dry, and you can start giving them a light liquid feeding.
