Romanesco Care: Firm Soil, Steady Water and Feeding

The head is made out of leaf

A romanesco head is built entirely from what the leaves produced before it started forming. A plant carrying twenty large dark leaves at the moment it switches over gives a heavy, deeply spiralled head; a plant with a dozen small ones gives a flat green disc with the geometry barely showing. Feeding after the head appears changes almost nothing, because by then the plant is no longer making leaves to feed.

So from planting out until the head is the size of an egg, the entire job is growing leaf fast and continuously. That takes three or four months on this crop, which is longer than most gardeners keep paying attention. The plants that disappoint are usually the ones that were looked after keenly in the first month and then left to themselves through late summer.

Romanesco is greedier than cauliflower and considerably greedier than calabrese. It has a shallow, wide, fibrous root system that lacks the depth to find water in a dry spell and the frugality to cope with thin soil. On a light, dry plot it is the brassica that shows the shortage first.

Judge progress by new leaf rather than by height. Four large new leaves in a fortnight through midsummer is a plant on course. A plant that has grown taller without much new leaf is short of nitrogen, water or root room.

Firm ground and no rocking

Brassicas head best in firm soil, and romanesco is the most emphatic case because of the size it reaches. A fully grown plant is two to three feet (60 to 90 cm) across and carries a heavy head on one stem in autumn, when the wind gets up. If the root plate is levered loose, the plant stops growing at exactly the point when it should be filling its head, and the result is a small, coarse one.

Prepare the bed a fortnight before planting so the soil settles, and do not fork it over again just before you plant. Then keep firming as the season goes on: earth up the stems at each feed and stake anything on an exposed site.

1. Let the bed settle before planting Dig in compost two or three weeks ahead and then leave it alone. Tread it lightly if it is fluffy. Planting into freshly turned soil guarantees a loose plant by autumn.
2. Earth up two inches (5 cm) at the first feed Draw soil up around the stem with a hoe. It stabilizes a top-heavy plant and encourages roots to form along the buried stem section.
3. Earth up again at the second feed Another two inches (5 cm) in late summer, before the plant reaches full size. By then the canopy makes it awkward to get at the stem.
4. Stake on windy or exposed sites A cane and a soft tie at about eighteen inches (45 cm) is enough. It costs a minute per plant and prevents the commonest late-season failure.
5. Refirm after storms Walk the row after any gale and press down any plant that has opened a gap in the soil around its stem. Water running into that gap rots the stem base.

Take hold of a leaf and pull gently. If the leaf tears before the plant shifts, the plant is firm. If the whole plant rocks, firm it and stake it today.

Watering: even, not generous

Romanesco wants about an inch (25 mm) of water a week, and appreciably more in hot windy weather when a canopy that size is transpiring hard. What matters more than the total is the evenness. A cycle of drought followed by a soaking is what produces a head that has lost its spiral, that splits along the stem, or that goes granular before it fills.

Wilting in the middle of a hot day is not in itself a problem and a large brassica canopy does it routinely. Wilting in the early morning is a real signal, and means the plant went short overnight. That is the point to water, not the point to wait and see.

1. Water at the base, not over the leaves The canopy funnels water outwards past the root zone. Aim the can at the soil around the stem and let it soak in before adding more.
2. Give a gallon (4 to 5 liters) per plant, twice a week One thorough soak beats daily splashing. Light frequent watering keeps the roots at the surface, which is the layer that dries first.
3. Test by digging, not by looking Push a finger two inches (5 cm) into the soil beside the plant. A dark surface after a shower means nothing; the root zone underneath can still be dust dry.
4. Mulch two inches (5 cm) once the soil is warm Compost or straw around the plants, kept an inch clear of the stem, halves the swing in soil moisture and removes most of the weeding.
5. Increase sharply as the head starts to show The fortnight in which the head expands is the peak demand of the whole season. A dry spell now costs head size directly and coarsens the surface.

Autumn rain is not a substitute for watering through a dry autumn spell. Check the soil under the canopy after rain, because a canopy this large sheds a surprising amount of it straight off the edge.

Feeding for leaf, then stopping

Romanesco wants a lot of nitrogen and it wants it early, through the leaf-building months. Work well-rotted compost into the bed before planting, then top dress twice while the canopy is going up. Once the head is visible, stop feeding entirely: late nitrogen gives soft growth, a loose open head and leaves pushing up through the spiral.

Potash matters more than people expect on a plant that stands this long. Floppy leaves on a plant that never firms up usually mean a soil short of potash rather than nitrogen. Aim for a near-neutral soil at pH 6.5 to 7.0, both because brassicas prefer it and because it keeps the trace elements available. Boron shortage on acid ground gives hollow brown stems and a bitter, flecked head, which is easy to mistake for something else.

Scatter top dressings in a ring six inches (15 cm) out from the stem and hoe them in shallowly. Fertilizer against the stem scorches it, and deep hoeing cuts the surface roots you are trying to feed.

Weeding without cutting the roots

Romanesco roots run wide and shallow, occupying the top three or four inches (8 to 10 cm) of soil right out to the edge of the canopy. That is exactly the zone a hoe works in. Weeding the bed deeply in midsummer is a self-inflicted check, and one that gardeners repeat every year without ever connecting it to the disappointing heads in autumn.

The answer is to get the weeds early while the plants are small, then mulch and stop. Once the canopy closes, a romanesco shades the ground so thoroughly that weeding becomes unnecessary anyway.

1. Hoe shallowly, and only in the first month Skim the surface at half an inch (1 cm) while the plants are small and the roots have not yet spread. After the first month, mulch instead of hoeing.
2. Hand pull anything within the canopy Inside a foot (30 cm) of the stem, pull weeds by hand when the soil is damp so they come out whole rather than snapping and taking roots with them.
3. Mulch rather than weed from midsummer Top up the mulch to two inches (5 cm) in midsummer. It suppresses the weeds, holds the moisture and keeps the hoe out of the root zone.
4. Strip yellowing lower leaves in autumn Dying leaves lying on damp soil start the soft rots that then move into the stem. Snap them off cleanly and take them off the plot.

Reading the plant through the season

Several different shortages make a romanesco look small and pale, so the useful question is which leaves are affected first and whether the new growth is normal in shape. New growth that is the wrong shape points at a trace element; old growth that is the wrong color usually points at nitrogen or water.

Write down the date each top dressing went on and the date the head first became visible. Over two seasons that pairing tells you whether your feeding is finishing before heading starts, which is the single adjustment that most improves the head.