How to Sow Arugula: Depth, Spacing and Soil Temperature

It germinates fast, or it does not germinate at all

Arugula is one of the quickest things in the garden to come up. In soil between 45 and 65F (7 to 18C) you should see the first hooked seedlings pushing through in four to seven days, and a full stand within ten. If a row is still bare at day twelve, something is wrong with the sowing, not with the seed.

The failure mode almost nobody expects is heat. Above about 80F (27C) germination drops off sharply, and above 85F (29C) a large fraction of the seed simply refuses. Gardeners read the packet, see that arugula is easy, sow into a sunbaked midsummer bed and conclude the seed was old. It was the soil temperature.

Buy a cheap probe thermometer and push it two inches (5 cm) into the bed in the middle of the afternoon, which is when the soil is hottest. That single reading tells you whether to sow now, sow into shade, or wait a fortnight.

Arugula seed keeps its vigor for three to four years in a cool dry jar, longer than most brassica relatives. Do not throw out last year's packet on the strength of one poor sowing.

Depth and spacing

The seed is small, roughly the size of a pinhead, and has very little stored energy. Sow it at a quarter of an inch (6 mm) and no deeper. Half an inch is enough to lose a third of the stand on a heavy soil that crusts after rain.

How you space it depends on what you want from the bed. A dense broadcast stand gives you small tender leaves for cutting, and you accept that you will harvest it once or twice and clear it. Rows with real spacing give bigger plants that recover from cutting and keep producing for weeks.

Firm the seed down with the back of a rake or the flat of your hand rather than burying it. Arugula needs contact with moist soil far more than it needs a covering.

Thinning is the step people skip

A crowded stand of arugula bolts earlier than a thinned one. Plants that are touching compete for water at the exact moment they are deciding whether to keep making leaves or run up a flower stem, and drought stress is the trigger that pushes them over.

Thin once, at the first true leaf, and be ruthless. The thinnings are the best salad you will eat all week, so nothing is wasted.

1. Wait for the first true leaf The two rounded seed leaves come first. Do not touch the stand until the first proper toothed leaf appears above them, usually eight to twelve days after emergence.
2. Water the row first Pulling a seedling out of dry soil drags its neighbors' roots with it. Water an hour beforehand so the soil releases the root cleanly.
3. Pull the crooked ones Keep the seedlings that stand upright with short thick stems. Remove anything leggy, pale, or leaning, and anything growing within an inch of a keeper.
4. Firm the survivors back down Press the soil gently around the plants you kept. Thinning always loosens roots, and a plant left with an air pocket at the collar wilts on the next warm afternoon.

Sowing when the soil is too warm

You do not have to give up on arugula in high summer, but you have to work against the heat rather than ignore it. Every one of the tactics below is aimed at the same thing: getting the top two inches of soil under 75F (24C) for the four days the seed needs.

Sow in the evening rather than the morning so the seed spends its first hours in cooling soil, and expect to water every day until the stand is up.

1. Water the bed the day before Soak the bed thoroughly twenty-four hours ahead. Wet soil holds a lower temperature than dry soil through the afternoon, and the moisture is already down where the roots will go.
2. Shade the row from the moment you sow Lay shade cloth of about thirty per cent, or a plank propped on bricks, directly over the sown strip. Take it off as soon as the seedlings are up, or they will stretch.
3. Sow a little thicker Expect to lose seed to heat, so increase the rate by roughly half. If more comes up than you planned, thinning solves it.
4. Start in modules in the shade instead Sow three seeds per cell in a tray kept somewhere cool and bright, thin to one, and plant out at three true leaves. Arugula transplants well as long as you do it young.

Why a sowing failed

Work backwards from what you can see in the bed. Arugula gives you fairly clear evidence, because it either comes up quickly or it does not come up at all, and the pattern of the gaps tells you which cause to blame.

Write down the soil temperature and the depth you sowed at every time. After three or four sowings you will know your own bed's cutoff far better than any general figure, and you will stop resowing into soil that was never going to work.