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GREEN-Seeds.com
vegetables & herbs
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FARMER'S BOOKSHELF
An information system of crops in Hawaii
Department of Horticulture
University of Hawaii at Manoa
LETTUCE
Climate
All
kinds of lettuce are adapted primarily to cooler grawing conditions. Heading
types are particularly sensitive to temperature and will rarely perform
satisfactorily when temperatures are higher than 75 F. Even for those
types of lettuce which perform satisfactorily at low elevations in Hawaii,
growth is generallyy better during the cooler periods of the year. Symptoms
of growth under too high temperatures are loose heads in the head types
and tipburn, early bolting (flowering), and bitteness in all types.
Cultivars
The
most common variety grown in low elevation areas is called Manoa in Hawaii,
but is the same as the variety Green Mignonette sometimes found in catalogues.
It is classed as a semihead variety, but the size and firmness of the
head varies considerably with the temperature under which it is grown.
The variety Anuenue, developed by the Hawaii Agricultural Experiment Station,
is also sometimes grown in the lowlands. It has a smoother leaf than Manoa
and will form a larger head when conditions are suitable. Only at higher
elevations (Kula on Maui, Volcano and Kamuela on Hawaii) can true head
lettuce be grown reliably. The variety grown is usually one of the Great
Lakes strains.
Many
varieties of leaf lettuce are also available. Three which were slower
to develop bitterness in trials at low elevation were Buttercrunch, Bibb,
and Parris Island Cos (Romaine type). Other varieties may also perform
well if they are harvested when young of if the weather is slightly cooler
than normal.
Culture
Irrigation
Lettuce
is very sensitive to moisture supply and should never be allowed to dry
out. Warm temperatures and low moisturesupply cause tipbumthe drying and
browning of the tips of the leaves. Irrigate in the morning before the
sun becomes hot, so that the pl ants will dry out rapidly and diseases
will not spread.
Diseases
 The most common diseases
of lettuce are tipburn, drop bottom rot, and spotted wilt virus. Tipburn
is a physiological disease and can be controlled only by choice of variety,
increasing soil calcium supply by liming highly acid soils, slowing growth
through lighter fertilizer application, and keeping an ample and uniform
supply of soil moisture present.
Drop and bottom
rot are caused by soil fungi. The symptoms are slimy rotting of the lower
leaves, then of the stem and the whole plant. Control is difficult, but
keeping the foliage dry and the fertilizer level low so that growth is not
overly succullent will help.
The symptoms of
spotted wilt are many tiny spots on the younger leaves, and stunted plants
which fail to develop and then rot. The virus is transmitted by thrips.
Control measures are a regular spray program to prevent a build- up of thrips,
and roguing of infected plants to prevent further spread of the disease.
Fertilizers
Nutrient needs
Lettuce does best in
well drained, slightly acid to neutral, high organic matter content soil.
On most Hawaiian soils lettuce will show a good response to an application
of lime and ample organic matter worked to a depth of 6 to 8 inches.
Lettuce is also
very susceptible to root knot nematodes. If nematodes are present, the soil
should be treated with a nematicide before planting. Symptoms of nematode
infestation are the presence of bumps on the roots and poor growth with
early wilting in warm weather.
General garden
fertilizer (like 10-30-10) applied at a rate of 1000 to 1500 pounds per
acre (about 3 to 3.5 pounds per 100 square feet) should be sufficient for
a crop of lettuce on most soils. Apply one-half the fertilizer at seeding
and the other half 3 to 4 weeks later.
Harvest
As the plants grow,
they may be harvested for use of the leaves until they have been thinned
to one plant every 8 to 10 inches in the row.
Lettuce in the home
garden can be harvested almost continuously for the leaves as soon as it
is large enough, and can be thinned by this method. Manoa and Anuenue mature
in 50 to 60 days; head lettuce and Romaine take about 65 to 85 days. Al
types will m ature faster when the temperature i high and slower when the
temperature is low. Har vest before bolting starts or bitterness develops.
Insects
The most common insect
pests of lettuce are cutworms, aphids, thrips, leaf miners, and garden loopers.
Cutworms eat young seedlings. To control, spray the ground with Malathion
or a similar insecticide before the seedlings emerge.
The other pests
feed on the leaves of the growing plants and are best controlled by a regular
spray program designed to prevent any buildup of these insects. Malathion
or naled (Dibrom) are generally effective, moderately toxic insecticides.
Slugs or snails can be adequately controlled by metaldehyde bait around
the lettuce planting.
Planting
Spacing single seeds
at 8-10 cm intervals is desirable but difficult with the small, irreglarly
shaped seed. One common technique is to count the seeds to make them larger,
rounder, and more uniform in shape. One common technique is to coat the
seeds to make them larger, rounder, and more uniform in shape. The coating
material is generally diatomaceous earth or montmorillinic clay (seed pelleting
common in California)
Lettuce is generally
seeded directly in the field but may also be transplanted. Plant the seed
1/4 to l/2 inch deep in rows 8 to 10 inches apart for the leaf or semihead
type, and 12 to 15 inches apart for the head types. As the plants grow,
they may be harvested for use of the leaves until they have been thinned
to one plant every 8 to 10 inches in the row.
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Green - Seeds Co., Ltd.
81/10B Ho Van Hue Street, Phu Nhuan District, Ward 9, Ho Chi Minh City, Vietnam
Tel: +84 (8) 847 6901 - Fax: +84 (8) 844 1392 - Email: info@green-seeds.com
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