LETTUCE CULTURE

New Zealand Journal of Agriculture, Volume 79, Issue 3, 15 September 1949, Page 237

LETTUCE CULTURE

LETTUCE, the most popular salad vegetable, has a place in most home gardens and is grown extensively by market gardeners. It is, however, sensitive to both unfavourable climatic and soil conditions, and the precautions necessary in its culture are outlined in

this article by

W. G. Crawford,

Agriculture, Dunedin.

LETTUCE thrives best under com-paratively-cool climatic conditions and requires ample moisture, two factors which have localised commercial production for early and main cropping, as low temperature and moderate growth are essential for good heading. Two main plantings or sowings are made commercially—one during early winter (just before frosts are experienced) and the other in early spring. When young, lettuces will stand fairly - heavy frost, but when approaching maturity are readily damaged. Soil Preference Though lettuce is grown on practically all types of soils ranging from clay loams to sandy loams, best results are usually obtained from fertile loams that are supplied with plenty of organic material and are well drained but able to retain moisture, as the plant has a relatively-small root system. Sandy loams are generally considered ideal for early production and the heavier type of soil is preferred for main cropping. Thorough preparation of the soil is essential for successful production. Drainage, tillage, harrowing, rolling, and later levelling of the land into beds when necessary are important. The thoroughness of the preparation of the soil before planting or sowing will largely determine the ease and efficiency of later cultivation.

Vegetable Instructor, Department of

Reaction of soil for lettuce should range from pH 5 to pH 6.7. The amount of lime applied should not exceed the requirements of the soil. Manures and Fertilisers Having a small root system, the lettuce plant is a relatively-poor forager, and it is therefore necessary for the surface soil to be well supplied with plant nutrients and humus. . On land in which fertility has been well maintained and where soil-improving crops such as lupins and oats have been grown lettuce may be grown as a catch crop, using the residual nutrients in the soil. However, when lettuces are grown in this way there must be sufficient nutrients in the soil, as an inadequate supply reduces both quality and yield. Fertilisation practices are best planned in accord with local advice and experience of previous crops, grown in the soil. Where it is available natural manure can be used to advantage, but artificial manure should; be used to supplement it. When soilimproving crops only are used as a means of humus supply applications of artificial nutrients should be greater. Light, sandy soils generally require larger applications than heavier types. When composted and properly rotted down before being applied to the soil the best fertiliser for lettuce is probably farmyard manure. In this condition it can be spread over the surface

and lightly harrowed in. If used fresh and unrotted, the material is coarse and interferes with planting or sowing and inter-row cultivation of the crop. Farmyard manure not only supplies plant food, but its organic material lightens heavy soils, thus improving aeration and water penetration, and increases the water-holding capacity of light or porous soil.

When manure cannot be obtained in sufficient quantities to supply both the plant food and organic matter green manure crops should be used in conjunction with commercial fertilisers. The green manure crop chosen should be capable of producing a good bulk under local climatic conditions and should preferably be a legume.

No hard and fast fertiliser programme can be recommended owing to the wide range of soil types in different districts, but in general it is a lack of available nitrogen in, the soil that usually limits the growth of lettuce plants. Nitrogen can be supplied in the form of nitrate of soda, sulphate of ammonia, or in a slower-acting form such as blood and bone.

A complete fertiliser that has been used with success is made up of 6 parts by weight of blood and bone, 5 parts of superphosphate, and 3 parts of potash. When mixed this fertiliser is used on manured soil at the rate of lOcwt. per acre and on unmanured soil at the rate of 15cwt. per acre. Sulphate of ammonia or nitrate of soda can be applied as a side dressing after the plants have been thinned and until hearts start to form. Late applications of this quick-acting fertiliser often result in the heads of lettuce bursting open.

Bed and Row Cultivation

The three main systems for growing lettuce commercially in the field are the bed system, the single-row system on the flat, and the single- or doublerow system on ridges.

The bed system is very popular with commercial growers, particularly

those who grow crops for harvesting in early spring and late autumn. Raised beds are usually made 3 to 4ft. wide, depending on the type of machinery used, and several rows of lettuce are planted 8 to lOin. apart according to the variety grown. The beds and rows run in the same direction as the water flow of the land, or north and south where practicable. This bed-and-furrow system facilitates drainage and makes for better aeration about the plants. In addition, the extra headlands provided by the furrows between the beds enable the crop to be harvested more conveniently.

The single-row system on the flat, which is the orthodox method of planting in single rows, is used for the growing of main crops of lettuce for warm-weather harvesting. The rows are standardised according to the cultural implements used. Where large areas are grown plenty of headlands should be available at the harvesting period and for this reason breaks should be provided between every 30 or 40 rows of lettuce.

The single-row system on ridges is also adopted for the growing of early and late crops and provides the same advantages as the bed system. The ground is ridged as for potatoes. The ridges are firmed, generally with a turnip roller, and one or two rows of lettuce either sown or planted on the ridge.

Growing Plants for Transplanting In the south lettuce plants suitable for autumn and summer transplanting are all grown in open beds and those intended for early-spring planting are raised under glass, usually in frames. The seed-bed should be well prepared. The soil should be in good physical condition, friable, capable of holding moisture, not subject to crusting, and free of diseases. The seed should be sown thinly in rows 12in. apart. As the seed is small and relatively weak in comparison with cabbage and other similar types of seed, care should be taken to make the sowing as shallow as possible, with just sufficient soil to ensure germination. Thick seeding is not only expensive, but is detrimental to the production of good, healthy, and sturdy plants. Lettuce plants require a constant supply of moisture, but care should be taken to see that overhead watering is not overdone, as this is often the cause of plants being thin and spindly, particularly when grown under glass.

Planting Out The transplanting of lettuce can be done in most districts at any time, provided the soil is in suitable condition and the plants have been properly hardened off when grown under glass. Only the best type of plants should be transplanted in the field, and the seedbed should be thoroughly soaked before lifting the plants. Planting is all done by hand, usually with a trowel when the single-row system on ridges is used for transplanting, or a shallow furrow may be made with a hand cultivator when using the bed or singlerow system on the flat. Lines should be perfectly straight to allow for inter-row cultivation of the crop, rows generally being from 15 to 20in.

apart, with 8 to 12in. between plants, The spacing will usually depend on the variety of lettuce grown, but overcrowding should be avoided. When planting care should be taken to have the crown of the plants level with the surface of the soil. Seed Sowing in the Field For practically all autumn and summer cropping the seed is sown direct in the field. Sowing is done with either single, hand-operated, or double or more' seeding drills operated by a small garden tractor. One and a half to 21b. of seed is sufficient to plant an acre with rows 14 to 20in. apart, but if the rows are closer, more seed is required. Larger areas than can be comfortably managed should not be sown at any one time. Successive sowings of small areas extend the marketing period of the crop and provide a better distribution of labour in the field. . I running Thinning is an important part of management and should not be neglected, as overcrowding leads to weak, spindly plants, which seldom develop into a good crop. Some growers as the first operation in thinning use a short-handled hoe to block out the

rows and then complete the work by hand. The spacing is similar to that of transplanted lettuce. Ciii+iva+ion vuipvaiiui: As lettuce, being a shallow feeder and generally a poor forager, cannot compete successfully with weeds,s, the crop yield is usually increased by cultivation. Weed destruction is of prime importance in cultivating lettuce crops, but as the root system of the plant is small and close to the surface, hoeing should be done carefully. The soil should not be drawn up to the plants unnecessarily, as this tends to obstruct the air circulation around them and encourages diseases. If the soil has been properly prepared before sowing or setting out the plants,'

only very shallow hoeing is necessary. Soil moisture should be conserved by the maintenance of a mulch, Varieties . , . ± Lettuce varieties are classified into four main typescrisp heading, cos, butter, and loose leaf, ZeaTmd ?hS ”ny othe? vaTtetv Mown Zealmtl thn ay other variety grown. ,9, os lettuce has long, narrow leaves with a pronounced midrib. The heart is compact and upright, but it is not hard. It is of little commercial value in comparison with the crisp-heading types. Butter' varieties are generally more flat or low lying than either the crispheading or cos types and are of very little commercial importance in New Zealand. > The best-known variety of this group is Big Boston. Loose-leaf types are generally vigorous growers, but are of little commercial value. The leaves are green with frilled edges. Varieties which have grown well over a number of years include: Webbs Wonderful, Success, Neapolitan (summer and winter market), New York, Imperial, Cos White, and Champion Cos.

Diseases Though lettuce crops are attacked by many diseases, only few are of sufficient importance to cause serious loss in the crop, but they are most difficult to control and some are beyond control by sprays. In fact, there are no truly-reliable control measures for any of them, as the nature of the growth of the plants makes it very difficult to apply the usual fungicides, _ ... "OtrytisKot . Botrytis rot (Botrytis ctnerea), which is also known as grey mould rot, is not of great importance, but is usually more serious in crops grown under glass. Plants may be attacked

any time from the seedling stage to when they are approaching maturity in the field. As high humidity favours the disease, preventive measures for plants grown under glass include regulating temperatures and ventilation and keeping the soil as dry as possible. Infected plants should be gathered and destroyed immediately by burning. Spotted Wilt Virus : There is no known control measure for spotted wilt virus apart from removing affected plants. Affected plants become stunted and are often misshapen. Outer leaves show irregular brown spots which later spread to the heart leaves. Collar Rot Plants affected with collar rot (Sclerotinia sclerotiorum) rot at ground level as the result of the fungus.

Symptoms when the plant is first attacked when very young resemble a common damping off. The disease on larger plants appears as a soft, darkened area on the base of the stem which develops rapidly under moderate temperature conditions. The optimum temperature at which this disease flourishes ranges between 60 and 70 degrees F. . When the head has rotted off small, black sclerotia off the fungus may be found among the decayed mass. Infected plants should be removed and destroyed, care being taken to collect sclerotia. Downy Mildew Downy mildew (Bremia lactucae) is one of the principal and most serious diseases with which commercial lettuce growers have to contend and heavy losses occur yearly in crops affected with this disease. It may attack the plant during all stages of growth and is of particular importance because it

affects the . edible leaves of the plant and ... during transit .. may predispose them to slimy ; rot. - The first indications ’of the disease are slight yellowish patches on the upper sides of the leaf. A white mould soon appears below these coloured areas, on the under sides of the leaf, which later turns brown. With glasshouse crops, preventive measures should be taken to keep the foliage as dry as possible and temperatures low. Both glasshouse and outside crops should be sprayed with Bordeaux every 14 to 21 days, commencing in the seedling bed. . .

Ring Spot

Ring - spot; (Mars,sonina panattoniana) is a fungus disease appearing as small brown spots on the bases of leaves, giving the plant a shot-hole appearance. . The affected tissues dry up and drop out. On the midrib of the leaf the sunken areas are irregular in outline. Control measures consist of^spraying. plants with Bordeaux at 2lnlay intervals. until hearts begin to form. ~ .

Silver Birch for Orchard Shelter

ORCHARD shelter is often a problem for fruit growers, because some of the best shelter trees, such as the poplar, rob fruit trees for 2 or 3 rows from the protective belt, sucker badly, and tend to grow too high, so that topping is difficult work. Two Hawke's Bay orchardists have used silver birch (Betula verrucosa) as shelter, with very satisfactory results. The photograph shows a silver birch hedge 35 years old which was side trimmed from the outset. For the past 20 years each side has been trimmed alternately every 2 years. The belt was first topped when 10 years old and has been topped 4 times sincetopping is necessary about every 7 years. There are no breaks in this shelterthe trees were planted 4ft. apartand it is very well furnished to ground level. It is sufficiently open to allow wind to drift through, which prevents any backwash caused by shelter that is too solid,, such as Cupressus lawsoniana. Silver birch does well on poor soils and is very hardy. Though it is a host for silver leaf, most deciduous trees are subject to that disease. A feature of silver birch hedges is that fruit trees can be grown dose to them without being robbed of plant food or moisture. The row of pears to the left of the photograph, though only 24ft. from the hedge, have grown and cropped as well as the second row. from the shelter belt. —R. I. JUNE, Orchard Instructor, Department of Agriculture, Hastings.