Hardening off and transplanting seedlings without the shock

Seedlings raised on a windowsill or in a greenhouse have lived a sheltered life. Plant them straight into full sun and wind and they can scorch, wilt or stall for weeks. Hardening off is the gentle in-between step that gets them ready.

Why hardening off matters

Indoors, seedlings get soft light, still air and steady temperatures. Outdoors, they face strong sun, drying wind, cool nights and hot afternoons. Hardening off gradually exposes them so their leaves toughen, their stems thicken and they learn to cope with losing water.

Punnets from a nursery are often already partly hardened, but if they've come from a sheltered shop shelf, a few days outside before planting still helps.

A simple routine

Over one to two weeks, increase their time outside bit by bit.

1. Days 1 to 2: an hour or two outside in a sheltered, shady spot, then back in.
2. Days 3 to 4: a few hours outside, with a little gentle morning sun.
3. Days 5 to 7: most of the day outside, including some sun, but out of hot afternoon sun and strong wind.
4. Days 8 to 10: outside all day and, if nights are mild, overnight too.
5. After that: they're ready to plant.

Check them often. Small pots dry out fast outdoors, sometimes twice a day in warm weather. If leaves look bleached or crispy, pull back to more shade for a day or two.

When to transplant

Timing makes a big difference, especially in Adelaide.

• Plant in the late afternoon or on a cool, cloudy day. Seedlings then have the night to settle before facing the sun.
• Avoid hot spells. If the forecast has a run of hot days, hold your seedlings back in a shady spot and keep them watered. Planting the evening after a heatwave breaks is much kinder.
• Avoid windy days. Hot northerlies dry out new transplants quickly.

How to transplant

1. Water the seedlings in their pots an hour or so before planting.
2. Water the bed too, so the soil is damp but not muddy.
3. Dig a hole a little bigger than the root ball.
4. Squeeze the pot or punnet and ease the seedling out, holding the root ball or a leaf, not the stem.
5. Set it in at the right depth (see below), fill in and firm gently.
6. Water in well with a seaweed solution. Seaweed solution isn't a fertiliser in the usual sense, but it's widely used to help roots establish and reduce transplant stress.
7. Mulch around the seedling, keeping the mulch back from the stem.

Planting depth

Most seedlings go in at the same depth they were growing in the pot. There are a couple of exceptions.

• Tomatoes can go deeper. Bury the stem up to the first set of leaves, removing any leaves that would sit underground. The buried stem grows extra roots, giving a stronger plant.
• Leeks are also often planted deeply to get a longer white stem.
• Don't bury lettuce crowns. Lettuce, silverbeet and strawberries grow from a crown at soil level. Bury it and it can rot. Keep the crown just at or slightly above the surface.

Shade for the first few days

In late spring and summer, even hardened seedlings appreciate some protection while they settle. Drape shade cloth over a few stakes or hoops for the first three or four days, or prop up an old chair or a palm frond on the sunny side. Remove it once the plants look perky in the afternoon.

Transplant shock and what to do

Some wilting on the first afternoon is normal. Signs that a seedling is truly struggling include:

• wilting that doesn't recover by the next morning
• yellowing or scorched leaves
• no new growth for a week or more.

If you see this:

1. Check the soil. Too dry is common, but too wet suffocates roots too.
2. Add shade for a few more days.
3. Water with seaweed solution again.
4. Hold off on strong fertiliser until it shows new growth.
5. Be patient. Most seedlings bounce back within a week or two.

Crops that dislike transplanting

Some crops grow best sown straight where they'll grow.

• Carrots and parsnips have a long taproot. Disturb it and you get stunted, forked roots. Always sow direct.
• Radish grows so fast there's no point transplanting, and it resents it.
• Beans, peas, corn and cucurbits can be transplanted, but they don't like their roots disturbed. Raise them in toilet-roll tubes or cell trays and plant the whole thing.
• Beetroot is OK to transplant if handled gently while small. Each beetroot "seed" is actually a cluster, so you can tease the seedlings apart and plant them out.

When in doubt, check the plant library or the seed packet. If it says "sow direct", it means it.

After planting

Keep new transplants evenly moist for the first couple of weeks while roots grow into the surrounding soil. Then start watering more deeply and less often, so roots go down rather than staying near the surface. Patchmates' reminders can help you keep track of watering and feeding for each bed.