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A turnip is one of the shortest crops in the vegetable garden. Six weeks for a small root, ten for a large one, and only two or three weeks of that are spent doing anything that requires your attention. The whole thing fits comfortably into the gap between two slower crops, which is the main reason to grow it.
That speed cuts both ways. There is very little time to correct a mistake, and a week of neglect at the wrong stage is a serious fraction of the crop. The schedule below is organized around the four or five moments where a decision genuinely matters; everything else is watering and watching.
Turnips are a cool season crop. They grow best between 50 and 70F (10 to 21C), the flavor is sweetest when the roots fill in cooling weather, and they will not do well in the heat of high summer. That gives two natural sowing seasons at either end of the warm half of the year.
The two sowing seasons
Which season you sow in changes the risks completely, and it changes which variety type suits. Spring sowings race the heat and the bolting risk. Late summer sowings grow into cooling weather, which is the easier and generally better crop of the two.
| Sowing season | Sow when | Harvest | The main risk |
|---|---|---|---|
| Early spring, under cover | As soon as the soil works | Late spring | Bolting after a cold spell |
| Mid spring, outdoors | Soil above 50F (10C) | Early summer | Flea beetle on the seedlings |
| Late summer | About ten weeks before hard frost | Autumn | Heat at sowing; root fly |
| Autumn, under cover | As the nights cool | Early winter | Slow growth as light drops |
If you only sow once, sow in late summer. Roots that fill in cooling weather are sweeter, the pest pressure is falling rather than rising, and there is no bolting risk to manage.
Week by week
The table below is written for a standard crop grown at four inch (10 cm) spacing, which takes seven to eight weeks. A baby crop at two inch spacing runs about two weeks shorter; a storage crop at six inch spacing runs two to four weeks longer. Adjust the later weeks, not the early ones, because the seedling stage is the same length whatever you are growing.
| Stage | What the plant is doing | Your job | The decision |
|---|---|---|---|
| Sowing day | Seed taking up water | Sow half an inch deep, firm, water, cover with mesh | Final spacing, which sets the size of root |
| Days 3 to 7 | Seed leaves emerging | Keep the surface damp, check under the mesh | Whether germination was good enough to keep the row |
| Week 2 | First true leaves | First thinning to an inch apart, hoe between rows | Which plants to keep |
| Weeks 3 to 4 | Leaf growth, taproot going down | Second thinning to final spacing, one light feed, water evenly | Nothing further; do not feed again |
| Weeks 5 to 6 | Root swelling, shoulders appearing | Steady watering, mulch, cut a few outer leaves | When to take the first test root |
| Weeks 7 to 8 | Root at full size | Lift alternate plants, then clear the row | When to stop, before it goes woody |
The two thinnings in weeks two and three are the only jobs that cannot be postponed. Everything else in this table tolerates being a few days late; those two do not, and a row thinned two weeks late never produces properly shaped roots.
Keeping turnips coming all season
Because the usable window on any one sowing is only two or three weeks, a continuous supply comes from repeated short sowings rather than from one large one. This is easier with turnips than with almost anything else, because a ten foot (3 m) row takes five minutes to sow and feeds a household for a fortnight.
- Sow a short row every two to three weeks Keep the rows small and frequent. Two or three weeks between sowings roughly matches the length of the harvest window, so each row is ready as the previous one is finishing.
- Pause through the hottest weeks Stop sowing when daytime temperatures sit above 77F (25C). Germination becomes patchy, growth checks, and the roots that do form are hot flavored and prone to bolting. Start again as the nights begin to cool.
- Use turnips as a gap filler A bed cleared of an early crop and not needed for six weeks is a turnip bed. This is the best use of the crop's speed, and it keeps ground covered that would otherwise grow weeds.
- Keep the rotation honest Frequent sowings make it very easy to end up growing brassicas in the same ground continuously. Map out where each row goes so that no bed carries turnips, radishes, rocket or cabbage more than once in three years.
Bolting, and the end of the season
Turnips bolt when a plant that is past the seedling stage goes through a run of cold, then warms up again. That sequence tells the plant it has been through a winter, and it flowers instead of filling the root. It is the main reason early spring sowings fail, and it is the reason a sowing made two weeks later can be fine when the earlier one was not.
Once a plant has begun to run up, it will not go back to making a root. Pull it. The flower shoots, picked while the buds are still tight, are worth eating in their own right and taste like a mild sprouting broccoli, so the plant is not entirely wasted.
- Delay spring sowings rather than gambling If the forecast shows a cold spell after the seedlings would be up, wait. A sowing made a fortnight later catches up quickly because the soil is warmer, and it avoids the vernalising cold.
- Use cover for the earliest sowings A cloche or low tunnel over an early sowing both raises the soil temperature and smooths out the cold nights that trigger bolting. Ventilate on sunny days so it does not overheat.
- Clear the last of the crop before hard frost Light frost improves the flavor of a turnip. Hard freezing damages the tissue and the root goes soft on thawing, so lift the last of the crop, or mulch it thickly, before the ground freezes properly.
- Write down the date the first sowing bolted Note which sowing ran to flower and which did not. The cut-off is sharp and it is specific to your garden, and after one season of records you will know exactly how early you can safely sow.
Record the sowing and harvest dates of every row for one full season. Turnips are fast enough that a single year's notes give you a working calendar, where a slower crop would take three or four years to learn.