LETTUCE PRODUCTION - VARIETIES AND PROPAGATION

Every gardener or grower has produced some superb heads of lettuce; not quite so many can be sure that they can make weekly deliveries of x doz. heads, all of them of excellent quality. And yet that is what every grower must aim for. To achieve a reliable production of top quality lettuce it is necessary to develop a system that is suited to all aspects of growing and marketing situation, and also to be prepared to give constant attention to detail.

Varieties

Spring Protected Crops: we have grown the following Butterhead varieties for spring cropping: Ravel, Plavenos, Claret, Talent, Bellona, Polo and Sitonia. Plevanos has proved by far the most satisfactory variety for all but the first sowing (Dee and early Jan), and the last (April) sowing, consistently producing a heavy head under the variable conditions of the last ten years. It is a thin-leaved type, attractive to the consumer, and, from reports of other organic growers, seems to respond u ell to organic methods on a variety of soil types. With high temperatures the head becomes rather loose and there can be problems with tip-burn - thus the unsuitability for the last indoor sowing.

Ravel is an old standard short day variety which will work for early sowing, but it is slow to bulk up after the rosette stage, and produces an unattractive thick leaved type. We were quite dissatisfied with appearance and quality of Talent and Polo; Cleret and Bellona were preferable to Ravel for Dee/Jan sowing, though both are rather thick leaved (unfortunately almost all new varieties are).

Sitonia has been successful for late spring sowing. A less attractive head than Plevanos, it can stand high temperatures with few problems from tipburn - though it is still advisable to grow the crop rather wet if temperatures are high. Bellona may also be used for the end of spring/summer.

For Spring crops of crisp lettuce we have grown Aubade, Globe, Lakeland, and Saladin. Aubade produces a small head, with uneven germination causing big problems; Globe trialled in 1987 did markedly better than the other varieties, uniform germination and growth with few outside leaves and good heavy heads; Saladin, grown by many Irish Organic Growers as both a protected and outside crop is rather too large for protected crop, with troublesome large outside leaves; Lakeland somewhat smaller and neater than Saladin did reasonably well, but heads were less attractive than Globe.

Summer: with difficult outdoor growing conditions it can be worth doing small weekly tunnel plantings through the summer as an insurance against outdoor crop failures. Sitonia has so far proved the best Butterhead variety for this.

Autumn: Protected Plevanos can be used again for early autumn crops; for later crops (harvesting Oct - early Dec) the most important factor for the organic grower is resistance to Bremia, and, to a lesser extent, Botrytis. From this point of view Ravel is useless, Sitonia not much better, Hudson grows quite well, but produces a rather unattractive head; and Bristol so far has given very satisfactory results.

Rossane produces attractive heads that bulk up quickly for early autumn crops, and apparently has good mildew resistance.

Propagation

A good propagation system is utterly essential to the economic production of lettuce. The aim must be to produce batches of healthy, vigorous and uniform seedlings, ready for planting out exactly on the intended date. This, in turn, should result in a crop that can be cut over once at harvesting, saving a considerable amount of time and allowing the earlier planting of the following crop.

There is no "right" system for all situations; various means can be used to achieve the requirements of each clement of a propagating system. The important thing is to develop a system that is appropriate to the scale and situation of production, and that works.

Seed: The basic choice (once variety has been decided upon) is between natural seed and pelleted seed. Recently this has teen further complicated by seed merchants offering a number of different types of pellets, and natural seed with higher levels of germination and vigour.

Most organic growers will probably not be using a blocking machine for sowing, but even for sowing by hand, pelleted seed is considerably quicker than natural seed. The pill must be kept moist which usually means covering the blocks or modules with either polystyrene sheets or moistened corrugated paper but it is also vital that the covering is removed as soon as the seedling "puts down a leg" - failure to do this will result in drawn seedlings

Pellets or split-pills are considerably mono expensive than natural seed but are probably more economic in most situations A problem for the smaller producer who may only want to use a variety for two or three weeks in a production sequence is that most seeds merchants appear to be continually increasing the minimum quantity of seed of a particular variety that they will supply - a case for co-operation amongst growers

Compost

Currently the Soil Association Standards permit the use of non organic seed composts (until January 1990) but where possible it is preferable to use either a bought-in organic seed compost or to make up a suitable compost. Shamrock seed and blocking compost both conventional are easily available everywhere and are reasonably satisfactory - a common problem with both can be a too lush nitrogenous growth where temperature are high and irrigation adequate. Existing organic composts produced in Ireland have (at the time of writing) not received the Soil Association symbol. Reports and experience on the use of them vary, but it would appear that more work needs to be done in this area.

Trays, modules and blocks

Lettuce should always be sown in some way that allows for considerable control during the propagation stage A basic rule in organic production is to always plant out a crop rather than Sown in situ where this is possible because of the considerable advantages this gives in weed control. No one these days will use a seed bed to produce plants; the three principal alternatives being seed trays, modules and blocks (either soil or peat) There are various advantages and disadvantages attendant upon each and most growers use a combination of more than one

Seed Trays

Some boxes or trays involve very little expenditure, they used to be constructed of wood , but are not always plastic though it is possible to use some standard sued disposable container peach trays for instance The tray should be well filled with compost, firmed around the edges If the seedlings are to remain in the trays until they are planted out (at 2-3 leaf stage) the seed should be space sown, allowing at East 2 mm between seeds. Alternatively seed may be sown much more thickly (up to about 400 to a standard Ward tray ) if the seedling are to, be transplanted into blocks or modules as soon as the cotyledons have expanded Seed (natural seed is usually used in tray 5) should be covered with an even spread of fine compost or peat - up to 6mm depth)

The use of trays is economical with regard to compost and space. in the propagating house as well as involving little capital expenditure. However space-sowing of natural-seed is expensive in labour unless very generous spacing :~ given there will be some check due to root disturbance at planting out and: it is likely that a grower will consider any check unacceptable in protected crops

Transplanting seedlings into modules or blocks cannot be given to unskilled labour , it is also time consuming

Seed trays might suit the very small grower , one with limited propagating space or for outdoor crops.

Soil blocks : Soil blocks have become the standard mode of propagating in lettuce production or rather peat blocks. Larger growers use automatic-blockers while those of us producing on a small scale use a hand blocker and sow by hand If you are making your own compost an extra consideration is that it should ha\e a physical structure that will hold well in the shape of the block without being too compacted.

For lettuce block size ranges from 3.5 -to 5,5 cm. A 5 cm block has the advantage that planting out can be delayed until at least the 3 leaf stage without any 'check - something of considerable importance in early protected crops in unheated houses in our highly unpredictable climate Though obviously larger blocks are expensive in compost and space

The correct moistness of the block Is critical: too wet compost produces a compacted block that Is inimical to good root development too dry results in a block liable to disintegrate

Blocks can be made on large trays. on slates or other suitable flat and rigid boards Moving them around can be awkward It is important to avoid where possible gaps between blocks as they lead to drying out

Blocks grown plants are extremely quick to plant out and the job can be done by unskilled labour . Normally beds are marked out in some way that produces slight depressions the size of the block used and the blocks an simply planed in the depression - where this is done it reduce the amount of basal rot and assists in quick harvesting

Modules: Modules come in all shapes and sizes, some with peg boards to eject plants for planting out We have successfully used the 104 cell Hassey Tray (45mm cells) Modules are extremely quick to fill and sow'; an easier to move around than blocks and are, more economical in. compost than blocks However for lettuce planting out is considerably slower It is also difficult to keep trays well watered in hot weather. Modules are re-usable (if treated with reasonable care) but the initial cost is quite high.

Heat and Light During and After Germination:

The production of predictable vigorous and uniform seedlings requires a fairly controlled environment for germination and early growth. For most varieties of lettuce a temperature of 15-18 oC should result in germination within 3 days. Lower temperatures will give slower and often more uneven germination whilst anything above 21 o C may produce high-temperature dormancy. For much of the year reasonably satisfactory results can be obtained without using artificial heat but for sowing in the winter or early spring some controllable heat source is necessary.

Poor light levels result in drawn slow-growing seedlings. From late October until mid-February much of the Ireland frequently has low light levels- certainly in the West the it is unlikely that levels will be adequate for the production of healthy sturdy plants. It is therefore advisable to give some artificial illumination for any sowing made during this period. This may be given continuously or for twelve hours each day , it may be the total light source or supplement day light.

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