Gadenin日本語Get the appTwo triggers, working together
Arugula is a long-day plant, which means that lengthening days push it towards flowering regardless of how well you are treating it. Warmth accelerates the same process. Put the two together in early summer and a plant that came up three weeks ago will send up a flower stem before it has made a decent rosette.
This is why the same seed behaves so differently in different seasons. Sown into shortening days in late summer, arugula will crop for six or eight weeks. Sown into lengthening days in late spring, the identical seed gives you one cut and a flower stem.
On top of the day length and the heat sits a third factor you control completely: stress. Drought, crowding, root damage and starvation all shorten the time to bolting, sometimes dramatically. A well-grown plant in a hot week may hold on. A thirsty crowded one will not.
| Trigger | What pushes it | How much you control it |
|---|---|---|
| Day length | Days getting longer past about fourteen hours | None, but you choose the sowing date |
| Heat | Sustained days above 80F (27C) | Partly, through shade and mulch |
| Drought stress | Any visible wilting, even once | Almost entirely |
| Crowding | Plants closer than 2 in (5 cm) competing | Entirely, by thinning |
| Root disturbance | Transplanting late, hoeing too close | Entirely |
| Starvation | Thin poor soil with no organic matter | Entirely |
Cold does not trigger arugula into flowering the way it does with an overwintered biennial. A plant checked by frost simply stops, and starts again when it warms up. Heat and day length are the pair to plan around.
How fast it happens by season
It helps to know roughly how long a sowing will last before you make it, because that decides whether it is worth sowing at all. The figures below are the interval from sowing to a visible flower stem in an open bed with reasonable soil.
Notice how short the midsummer window is. A sowing that bolts at thirty days will give you one cut of baby leaf and nothing more, which is fine as long as you sowed it thickly and expected exactly that.
| Sowing season | Sowing to flower stem | Cuts you can expect | Verdict |
|---|---|---|---|
| Very early spring, cool soil | Fifty-five to seventy days | Three or four | Excellent, the second best window |
| Late spring, days lengthening | Thirty-five to forty-five days | Two | Worth doing, cut early |
| Midsummer | Twenty-five to thirty-five days | One | Baby leaf only, sow thick |
| Late summer, days shortening | Sixty to eighty days | Four or more | The best window of the year |
| Autumn under cover | Often not at all before winter | Picked over for months | Slow but very long lasting |
What actually delays it
None of these will hold a plant through a heatwave in midsummer. Together they will typically buy you ten days to a fortnight in shoulder seasons, which is the difference between two cuts and four.
Do them all, or do none. Shading a bed you then let go dry achieves nothing, because the drought stress replaces the heat stress you removed.
- Never let it wilt Keep the top six inches (15 cm) evenly moist from germination onwards. A single afternoon of wilting can pull the flower stem forward by a week and permanently coarsens whatever leaf follows.
- Shade the afternoon sun Thirty per cent shade cloth over the bed from midday onwards lowers the root zone by several degrees. Put it up before the hot spell arrives, not after the plants have already checked.
- Mulch between the rows An inch (2.5 cm) of compost or clippings holds moisture and keeps the soil cool. This is the single cheapest bolting delay available.
- Thin properly and early Give each plant two to four inches (5 to 10 cm). Crowded plants compete for water and bolt as a group, often a week or more before a thinned stand in the same bed.
- Sow into a shaded bed in summer Use the north or east side of a taller crop for midsummer sowings. The reduced afternoon sun is worth more than the light you give up.
- Sow small amounts every ten days Successions are the real answer. Rather than fighting to keep one sowing going for two months, accept that each will bolt and have the next one already coming.
Reading the plant before the stem shows
The stem is the last signal, not the first. If you learn the earlier ones you can bring your harvest forward by a week and get the good leaf instead of the tough leaf.
The changes below happen in roughly this order. By the time you can see a stem in the center of the rosette, the decision was made ten days earlier.
| Stage | What the plant looks like | What to do |
|---|---|---|
| Still leafy | Broad flat leaves, rosette spreading outward | Cut normally, keep watering |
| Early warning | New leaves narrower and more deeply cut than the old ones | Cut everything usable within the week |
| Center lifting | The middle of the rosette rises and tightens | Take the last full cut now |
| Stem visible | A stalk clears the leaves, buds clustered at the tip | Strip the usable leaf, decide whether to keep the plant |
| In flower | Four-petalled cream flowers with dark veining | Pick flowers for salad, or leave for seed |
Taste is the honest test. Chew half a leaf from the center of a plant: if the heat lingers at the back of your throat rather than fading, that crop is on its way out whatever it looks like.
A bolted plant is still useful
Do not pull the whole row the moment it flowers. The cream flowers with their fine dark veins are edible, considerably milder than the leaves, and they hold their shape in a salad. The young green seed pods, picked while still soft enough to squash between finger and thumb, taste of sharp mustard and are worth eating raw.
Bees and hoverflies work arugula flowers heavily, so a patch left to flower at the edge of the vegetable garden earns its space as insect food when little else is out.
If you leave a few plants to ripen their pods fully, they will shed seed and give you volunteer seedlings in the same bed. These self-sown plants come up earlier than anything you sow deliberately and are noticeably tougher, though the leaves are usually hotter. Let them run if you want an early crop, and hoe them out if you want the bed for something else. Either way, note in your records which bed you let seed, because you will forget by spring.