Itinerário técnico
Um guia de cultivo passo a passo, da sementeira à colheita.
Sow on the surface or at most half a centimetre deep, because the seed needs light to germinate and burying it is the commonest mistake. Germination works best between 15 °C and 20 °C and stalls above 25 °C, so in summer sow in the shade or chill the tray in the fridge for two days before bringing it into the light. Transplant at four or five true leaves without burying the collar, or sow direct and thin. Sow every two weeks for a continuous supply.
It wants loose, cool soil, rich in organic matter and free draining, at a pH between 6.0 and 7.0. The root system is shallow, so the top layer is what counts: keep it cool and never let it compact. A raised bed helps a lot in heavy soil, because lettuce rots at the collar in standing moisture.
It is the cool weather crop par excellence, between 15 °C and 20 °C. Above 25 °C the plant bolts, fills with latex and turns irreversibly bitter. Fifty per cent shade netting over the middle of the day greatly extends the picking season in a warm climate. This is a cool season crop and heat is its main enemy. The Portuguese coast offers close to ideal conditions and allows harvests over much of the year. Inland in Portugal and Spain, concentrate the crop in autumn, winter and early spring, because from June onwards heat and long days push it to bolt and turn bitter. In France it grows easily from spring to autumn, and in Germany the whole summer works because it is rarely too hot. In Italy follow the Iberian calendar in the centre and south, concentrating the crop in autumn and winter, and the French calendar in the north, where the Po valley summer rarely makes it bolt too early. Light shade over the middle of the day extends the harvest in any warm climate.
Because the leaf is the crop, nitrogen matters, but in moderate, slow release form. Mature compost before sowing and one or two waterings with liquid organic feed during growth give tender leaves without building up nitrates. Heavy late feeding makes the leaf brittle, attracts aphids and shortens shelf life after harvest.
Water often, a little at a time and always in the morning, keeping the soil moist but never waterlogged. Watering in the afternoon or evening leaves the leaves wet and brings downy mildew and botrytis. Water shortage speeds up bolting and makes the leaf bitter.
Hoe often and always shallowly, because the roots are short, near the surface and break easily. The best method is to prepare the bed two or three weeks in advance, let the first flush of weeds germinate, remove it and only then sow: that gives you a decisive head start. A thin layer of sieved compost between the rows greatly reduces later work.
Slugs and snails are enemy number one for this crop and can clear a whole sowing in one damp night. Use barriers of ash, eggshell or sawdust around the bed, collect them at night with a torch and fall back on ferric phosphate if needed. Aphids settle in the heart of the plant and are hard to wash out later: apply potassium soap at the first sign and encourage ladybirds and hoverflies with flower strips. Leaf miner draws pale tunnels in the leaf, which are simply removed by hand. Thrips and flea beetle attack mainly in dry warm weather.
Lettuce downy mildew covers the leaf underside with a greyish white bloom and is the classic problem of the humid Atlantic coast. Botrytis rots the collar in cool wet weather. Both are prevented the same way: generous spacing, morning watering at the base, a raised bed and transplanting without burying the collar. Sclerotinia forms a white cottony mould at the base and calls for a long rotation. Remove any diseased plant at once.
Cut early in the morning while the plant is still turgid, slicing through the base with a knife. In loose leaf varieties you can pick leaf by leaf for weeks, taking the outer ones and leaving the centre to grow. As soon as the plant starts to stretch upwards at the centre it is bolting: cut it at once, because within days it turns bitter. Allow about 45 to 55 days from transplanting to the first harvest of this variety. The reference colour is green, and that is what tells you it is ready.
