Broccoli, Cabbage and Cauliflower Seedlings

Broccoli, cabbage and cauliflower are the same kind of plant, and they are transplanted on a similar schedule: set out in spring while the weather is still cool, or raised again in summer for a fall crop. The seedlings are hardy compared with tomatoes and peppers, but they are also less forgiving of crowding, of being held too long indoors, and of rough treatment at planting time. A cabbage-family transplant that goes into the garden stocky, well hardened off and at the right age will usually head without trouble. One that is overgrown, stretched or shocked by cold may button, bolt or simply sit there.

When to sow broccoli, cabbage and cauliflower indoors

Maryland Extension puts cabbage, broccoli and cauliflower at 4 to 6 weeks indoors before they go outside. Clemson Extension’s table gives 5 to 7 weeks for broccoli, Brussels sprouts, cabbage, cauliflower, collard and kale. Either way, this is a short stay indoors compared with pepper or tomato, and sowing too early is a common way to end up with a poor transplant.

For spring, work backward from the last frost date. Maryland Extension sets broccoli transplants out about four weeks before the last spring frost date, so count the indoor weeks back from that planting date, not from the frost date itself. That date is given for broccoli; cabbage and cauliflower are also set out in cool weather, but cauliflower is less tolerant of cold than broccoli, so check the seed packet or the local Cooperative Extension office for their dates.

For fall, Maryland Extension gives about 4 to 6 weeks from seeding to a transplant for fall broccoli. Count backward from the first fall frost date rather than the last spring one, and allow for the 55 to 65 days broccoli needs from transplanting so the crop heads before hard cold.

Spring versus fall sowing

Spring transplants go out into cool soil and cool air, which suits them, but the crop has to size up before summer heat arrives. Fall transplants go out into warm soil and shortening days; Maryland Extension notes that fall broccoli often yields better than spring broccoli in the mid-Atlantic. If you are planning a second season, the timing and crop choices are set out in fall vegetable transplants: planning a second season.

Raising the seedlings: containers, mix and light

Treat these seedlings like any other vegetable transplant at the start. Use a soilless seed-starting or potting mix rather than garden soil, which is too dense in a container and may carry weed seeds and disease. Fill containers to within one inch of the top and moisten the mix before filling.

Sow about twice the seed’s diameter deep. A standard flat is 10 inches by 20 inches, and the suggested cell or pot size for broccoli and cabbage is 4 to 9 square inches per plant. Starting one seed per cell avoids crowding and makes potting up simpler.

Light is where cabbage-family seedlings go wrong fastest. A windowsill is seldom enough, and stems stretch and lean toward the glass. Keep fluorescent or LED fixtures 2 to 6 inches above the plants, on for 14 to 16 hours a day with an outlet timer, per Maryland Extension. Clemson gives 16 to 18 hours of bright light with tubes 2 to 4 inches above the seedlings, raised as the plants grow. Tomato and cabbage-family seedlings stretch readily when the light is too far away or the room is too warm.

Water when the top of the mix is nearly dry, and let it dry slightly between waterings. Seedlings in constantly wet mix are the ones that damp off at the soil line.

Potting up and holding them back

Seedlings started together in a flat move to their own containers shortly after the first true leaves appear. Hold them by a leaf, never by the stem. Leaving them crowded in the flat too long is one of the most common mistakes, and with such a short indoor schedule there is little time to recover from it.

If seedlings get ahead of the weather, cool conditions and good light slow them without harming them. Warmth and crowding push them into tall, soft growth that does poorly outside.

Hardening off is the step that decides the crop

Hardening off gradually adapts indoor-grown plants to sun, wind and cold, and for cabbage, broccoli and cauliflower it matters more than for warm-season crops set out after all danger of frost. Maryland Extension describes a 5- to 14-day period depending on the crop, beginning 1 to 2 weeks before planting out. Clemson starts once plants are about three inches tall and continues for two to three weeks.

Start in a shaded, wind-protected spot for part of the day, increase sun by an hour or two each day, and water less often but never let plants wilt. Do not put tender seedlings out on windy days or below 45°F. Bring cool-season crops such as lettuce and cabbage in if a freeze is forecast. A cold frame is an excellent place to harden off. The full sequence is in how to harden off seedlings before planting out.

Cold below 45°F: bolting and buttoning

Cool weather suits these crops, but there is a limit. Maryland Extension notes that temperatures below 45°F can stop heading or cause bolting in broccoli. Cauliflower is the most sensitive of the group: hardened too severely, it can form thumb-sized heads and stop growing altogether. That is the cost of overdoing hardening off.

A light frost is usually not the problem for a properly hardened broccoli or cabbage transplant. The problem is a long stretch of cold, or a seedling that was pushed hard and then held in the cold.

Planting out and spacing

Set transplants on a cloudy, calm day. Water them several hours before, dig a hole large enough for the root mass, firm the soil, and water in with a weak liquid fertilizer at label dilution. Shade and shelter new transplants for a few days with baskets, pots or floating row cover.

Good transplants are stocky with deep green leaves and white, fibrous roots. Avoid overgrown, woody plants with a thick mat of circling roots, and plants carrying pests.

Follow the seed-packet spacing for cabbage and cauliflower. Squeezing plants closer causes disease and lowers yield. Biodegradable pots can go in the ground, but remove the base and any rim above the soil line, because an exposed rim wicks water away from the roots.

Broccoli is rated hardy and withstands heavy frost once established. Days to maturity from transplanting run 55 to 65 for broccoli. If you are also raising warm-season transplants, the schedule for those is different; see growing tomato seedlings from seed to garden for the contrast in timing and handling.

What goes wrong with cabbage-family seedlings

• Sown too early. A long stay indoors gives tall, root-bound plants that do poorly in the garden. With only a few weeks needed indoors, there is no reason to start earlier.
• Too little light. Stretched, leggy seedlings come from fixtures too far away or a windowsill, made worse by warmth and crowding.
• Crowded flats. Seedlings left in the flat past the first true leaves compete for light and water and never make a stocky transplant.
• Over-hardening cauliflower. Hardened too severely, cauliflower can form thumb-sized heads and stop. In broccoli, temperatures below 45°F can stop heading or cause bolting.
• Late spring planting into heat. These crops need to size up in cool weather. A transplant that goes out late meets summer heat before it has headed.
• Wet mix and damping-off. Water molds need free water. Use a soilless mix, clean containers and let the surface dry slightly between waterings. Infected seedlings cannot be saved: remove them with their mix, wash the containers, soak them for 1 to 2 minutes in 1 part chlorine bleach to 10 parts water, and rinse.
• Fungus gnats. Tiny dark flies around pots are mainly a sign of overwatering. Let the mix dry and use yellow sticky traps to monitor. If a product is used for the larvae, read and follow the label.
• Rough handling at planting. Keep the root ball intact, and never hold a seedling by its stem.

A short season, done in the right order

Sow 4 to 6 weeks (Maryland) or 5 to 7 weeks (Clemson) before the intended planting date, give the seedlings strong light close overhead, pot them up before they crowd, and harden them off gradually over 5 to 14 days. Set broccoli out about four weeks before the last spring frost, keep an eye on nights below 45°F, and repeat the whole sequence in summer for a fall crop. The plants do the rest.