Lettuce is fairly tolerant of soil type but do well on light sand to sandy loams. As ettuce has a very shallow root system it can be grown on relatively shallow soils provided a good irrigation system is in place so favourable soil moisture can be maintained. Due to the shallow root system of lettuce, the majority of the roots only go to a depth of 300mm. Therefore, nutrient and water requirements need to be concentrated in this small volume of soil. If planting in soils that stain i.e. red soils, a mulch between the soil and the bottom leaves will keep the soil stain from spoiling the heads and making them difﬁcult to sell. Washing off soil stain causes bacterial rot so is not recommended. Soil PH should be between 5.5 – 6.
As lettuce is normally a cool weather crop demand for it in the cooler months is quite low. Consumer’s main demand for Lettuce is during the hot summer months when salads are very desirable.
Lettuce heads produced in summer receive a higher price than those grown in the cooler months. Summer production of lettuce can be done, however, some precautions need to be addressed to safe guard the viability of the crop. Varieties need to be chosen carefully for the cool winter production and hot humid summer production.
Summer varieties need to have a good tolerance to bolting and good disease resistance. Most varieties perform equally well in the cooler months so ask advise of a Prime Seed Agronomist who will give you the best variety for your planting programme. The most favourable temperature for production is a daily mean of between 15° C and 18° C. Lettuce however will perform in a range of 17° C – 27 °C during the day and 2° C to 12° C night temperatures, with careful variety selection.
For best results soils need to be prepared in advance and beds made up for summer production. Raised beds of at least 20cm – 30cm high need to be made to help drainage with excess rainfall. Planting on the ﬂat in winter is acceptable. Drip irrigation is the preferred method to irrigate Lettuce as there is no soil splash and soiling of the heads. If overhead irrigation is to be used a smaller nozzle size should be ﬁtted to the sprinkler head to avoid big droplets hitting the soil surface and splashing the heads with soil particles.
As soil moisture is an important factor in lettuce production the level of available water in the root zone should not get below 50 percent holding capacity before the next cycle. Careful planning of irrigation is necessary regarding soil types and the time of year.
Amounts of water to be applied vary from 18mm on sandy soils to 50mm on heavy soils per cycle. Lettuce needs lighter and more frequent irrigations more so than many other crops. Frequent irrigations however, especially in summer production favour conditions for development of several diseases.
The use of quality speedlings is highly recommended to achieve a good uniform crop. Prime Plants Nursery does Lettuce seedlings, both the frilly type and standard varieties. For planning purposes seedlings normally take about 5 weeks in the Nursery in the winter months to just under 4 weeks in the warmer months.
Seedlings must be well hardened before planting during the summer as they will be subjected to high temperatures in the ﬁeld after transplanting.
Mulching of summer production is deﬁnitely recommended. After transplanting the seedling a mulch using wheat straw, grass or saw dust is advisable. This will assist in lowering the soil temperature, cooling the crop, conserving moisture and preventing soil splash.
During summer because of heavy rainfall which cannot be controlled planting on raised beds is recommended. These beds can be made with a distance of 1,5m – 1,8m bed centre to bed centre. 3 rows marked out on the top of the bed at 30cm apart with seedlings placed 25cm in row, giving round 80,000 plants per hectare.
For larger size heads distance in row can be increased to 30cm giving a lower plant population in order to achieve a larger head size. On the ﬂat, rows can be between 35cm – 45cm apart and in row spacing between 25cm – 35cm. This gives a plant population of about 60 000 to 100 000 plants per hectare. Head size for speciﬁc large head orders use the wider spacing.
Get a soil analysis done before planting. A general recommendation is to use a basal dressing of Compound “B” at a rate of 500 – 600kg per hectare applied pre planting. If the fertiliser is cupped in by hand to a planting hole for a seedling make sure the fertilizer is well mixed in the hole with the soil to prevent root burn of the seedling.
This takes place about 8-10 weeks after transplanting. A visual inspection is done to check if the heads have reached the desired ﬁrmness and size. Loose leaf or frilly lettuce types are harvested when the leaves have grown to a reasonable size. Using a long blade knife cut the stalk just above the level of the soil and retain most of the wrapper leaves.
Harvest when the plants have dried out and in the cool of the early morning before the day temperature gets too hot and the plants begin to wilt. Do not harvest the heads just after rain or irrigation as this makes the leaves crisp and brittle because they will have absorbed a lot of water and the leaves will break easily.
After cutting, damaged, diseased and discoloured leaves can be removed and the cut end tidied up before placing in the crate. Do not over pack the crates and if possible not to stack more than 2 high. Move heads quickly once harvested into a ﬁeld shelter with wet walls to prevent dehydration before transport to the packing area.
Once at the pack shed storage under refrigeration with a temperature of 0°C and a R.H. of 95 percent lettuce heads can be stored for up to 3 weeks if there is a glut in the market .
However quicker movement to market is preferable. If there is no refrigeration the crop will have to be moved to market the same day it is cut. Lettuce heads can weigh from 500g to over 1,2kg depending on the time of the year. Yields can be from 25 – 30 tonnes per hectare. If properly grown lettuce heads should be ﬁrm, fresh, clean and crisp and not have a bitter taste.
During transport to market if the heads are being delivered in an open vehicle it is very important to cover the lettuce heads with a damp cloth type cover to prevent wind burn and dehydration. If the delivery vehicle has an insulated covered back be sure that the temperature inside does not go too high as this can cause dehydration and wilting.