All plants

Lettuce

Lactuca sativa

Asteraceae · sunflower family

Matures
40–90 days
Harvest
Over a period · Annual
Sun
4–8 hrs
Spacing
4–12 in
Botanical engraving of Lettuce

Planting window

When to sow, by microclimate — foggier blocks run a different calendar than sunnier ones.

JanFebMarAprMayJun
Foggy●●●●●●
Sunny●●●●●●
JulAugSepOctNovDec
Foggy●●●●●●
Sunny●●●●●●
Well-suitedMarginalNot suited

Sunlight

How well it does at each level of direct sun per day.

<2 hrsNot suited
2–4 hrsMarginal
4–6 hrsWell-suited
6–8 hrsWell-suited
8+ hrsMarginal
Well-suitedMarginalNot suited

Soil depth

Root room it needs — useful for sizing raised beds and containers.

6–10"Well-suited
10–15"Well-suited
15–18"Well-suited
18–24"Well-suited
Well-suitedMarginalNot suited

Spacing

Room each plant needs — along the row and between rows — for sizing beds.

In-row
4–12 in
Between rows
12 in

Growth habit

The shape it makes in the bed and how tall it gets — what tells you whether to give it a trellis, a stake, or just room.

Habit
Low mound
Mature height
6–12 in

Low and self-supporting — no staking needed.

How to start it

Sow it where it grows, or raise it under cover and set it out — and how far ahead to start the seedlings if it's the latter.

Usually
Direct-sow
Start indoors
4 weeks ahead
Direct-sowWell-suited
TransplantWell-suited

Sown straight into the bed it will grow in.

Well-suitedMarginalNot suited

Getting started

How it goes in the ground — how deep to sow or set it, and the soil temperature it needs to wake up. The cold-soil floor is the real limiter for direct-sowing in a foggy SF spring.

Started from
From seed
Ideal soil temp
60–75°F
Sow depth
0.25 in
Time to emerge
3–4 days

Germinates between 32–85°F; happiest around 60–75°F.

30°F110°F
IdealWorkable

Pests & diseases

What tends to trouble this crop — what it is, what you'll see, and what to do about it, least-toxic first.

Pests9

Aphids
What it is
Aphids have soft pear-shaped bodies with long legs and antennae and may be green, yellow, brown, red, or black depending on the species and the plants they feed on. A few species appear waxy or woolly due to the secretion of a waxy white or gray substance over their body surface.
What you'll see
Low to moderate numbers of leaf-feeding aphids aren't usually damaging in gardens or on trees. However, large populations can turn leaves yellow and stunt shoots; aphids can also produce large quantities of a sticky exudate known as honeydew, which often turns black with the growth of a sooty mold fungus.
What to do
Before planting vegetables, check surrounding areas for sources of aphids and remove these sources. On the other hand, these aphid-infested weeds can sometimes provide an early source of aphid natural enemies.
Source
Armyworms
What it is
Armyworm larvae feed in groups, which distinguishes them from other vegetable pests such as corn earworms and loopers. Markings on newly hatched armyworms are usually hard to distinguish from those of other caterpillars; older larvae have distinct lengthwise stripes.
What you'll see
Armyworms may feed on the crowns of seedlings. On larger plants, armyworm caterpillars skeletonize leaves.
What to do
Handpick. Virus diseases, parasites (Hyposoter and Trichogramma), and general predators may be effective on caterpillars.
Source
Corn Earworm or Tomato Fruitworm
What it is
The corn earworm or tomato fruitworm is a common caterpillar pest of many vegetables and flowers. Newly hatched larvae have several rows of dark bristles on their backs.
What you'll see
Earworms feed on leaves, buds, and flowers of many vegetable crops. They destroy seedlings, bore into lettuce heads and bean pods, make deep watery cavities in fruit, and leave frass.
What to do
Handpick, and avoid spraying with insecticides. Bacillus thuringiensis may kill 40 to 60% of the population but must be applied just after eggs hatch and before caterpillars enter fruit.
Source
Cutworms
What it is
Cutworms are dull brown caterpillars ranging in length from 1 to 2 inches when fully grown. They curl into a C-shape when disturbed.
What you'll see
Cutworms feed on blossoms and leaves of many ornamental plants and attack most garden crops. They clip off seedling stems near or just below the soil level.
What to do
Cutworms are pests mainly in the spring. Destroy crop residues; keep garden weed-free in winter.
Source
Flea Beetles
What it is
Their small size, enlarged back legs, characteristic shot-hole damage, and habit of jumping when disturbed make flea beetles easy to identify. There are numerous flea beetle species.
What you'll see
Adults feed mostly on leaves, leaving tiny pits or small holes at their feeding sites. Flea beetles feeding together can cover leaves with bleached, pitted areas or ragged holes.
What to do
Remove debris in the fall to remove any overwintering beetles. Reflective mulches applied very soon after planting and before plants become infected can be helpful.
Source
Loopers
What it is
Loopers are green caterpillars with several white stripes down their backs. They arch their backs as they crawl, this looping movement giving them their name.
What you'll see
Loopers eat irregular holes in leaves, most often older leaves. They may also bore holes in lettuce and cabbage heads or damage fruit.
What to do
Handpick. Apply Bacillus thuringiensis or spinosad; they are effective against loopers, especially when caterpillars are small.
Source
Snails and Slugs
What you'll see
Snails and slugs feed on a variety of living plants and on decaying plant matter. They create irregular holes with smooth edges on leaves and flowers by scraping with their rasp-like tongues.
What to do
It will not be possible to eliminate some shelters, such as low ledges on fences, the undersides of wooden decks, and water meter boxes, so make a regular practice of trapping and removing snails and slugs from these areas. The survivors congregate in the remaining shelters, where you can more easily locate and remove them.
Source
Vegetable Leafminers
What it is
Adult leafminers are small, active, black flies often with a prominent yellow triangle between the bases of the wings. The head behind the eyes is mostly black.
What you'll see
Leafminers attack many different vegetable and flower hosts, including cole crops, cucurbits, tomatoes, peas, beans, aster, begonia, dahlia, impatiens, lily, marigold, petunia, and verbena. Adult female leafminers puncture leaves and sometimes petals to feed on exuding sap.
What to do
Leafminers rarely require treatment in gardens. Clip off and remove older infested leaves.
Source
Whiteflies
What you'll see
Whiteflies use their piercing, needlelike mouthparts to suck sap from phloem, the food-conducting tissues in plant stems and leaves. Large populations can cause leaves to turn yellow, appear dry, or fall off plants.
What to do
Whiteflies have many natural enemies, and outbreaks frequently occur when these natural enemies have been disturbed or destroyed by pesticides, dust buildup, or other factors. General predators include lacewings, bigeyed bugs, and minute pirate bugs.
Source

Diseases6

Big Vein of Lettuce
What you'll see
Big vein disease is a viruslike disease that causes the veins in leaves to become very enlarged and clear. Such deformities are easily seen if the leaf is examined with the light source behind it.
What to do
Lettuce big vein is caused by a viruslike agent vectored by the fungus Olpidium. Avoid fine-textured, poorly drained soils where big vein has been a problem.
Source
Botrytis Rot (Gray Mold) on Lettuce
What it is
Symptoms of Botrytis crown rot are similar to sclerotinia rot: lower leaves of plants infected with Botrytis yellow, and eventually the whole head wilts and rots. A watery, brown decay develops on the underside of the head at the crown.
What you'll see
Leaves on or near the soil or in the dampest areas of the garden turn light brown and develop a gray or brown fuzzy growth of fungal spores. Plants shrivel and rot and often develop flattened hard black masses, called sclerotia, under rotted parts.
What to do
Plant on well-drained, raised beds, use furrow irrigation whenever possible, and try to keep the surface of vegetable beds as dry as possible. Remove diseased plants as soon as they appear and dispose of them.
Source
Downy Mildew on Lettuce
What it is
Downy mildew gets its name from the downy masses of spores it produces on undersides of affected leaves. These downy growths initially may be confused with those of powdery mildew.
What you'll see
Downy mildew causes light green to yellow angular spots on the upper surfaces of leaves. White fluffy growth of the fungus develops on the underside.
What to do
The best way to prevent downy mildew is to avoid the environmental conditions that favor it. Destroy infected crop debris and susceptible related weeds before planting a new crop.
Source
Powdery Mildew on Lettuce
What it is
Powdery mildew is often confused with downy mildew. Both downy mildew and powdery mildew produce whitish collections of spores on the leaf surface, but similarities end there.
What you'll see
Powdery mildew usually appears first as yellow spots on the upper leaf surface of older leaves; these spots develop the characteristic powdery growth and symptoms spread to the undersides of leaves and stems. Affected leaves may turn completely yellow and die.
What to do
Powdery mildew rarely becomes serious enough to require treatment. Powdery growth of powdery mildew.
Source
Spinach and Lettuce Virus Diseases
What you'll see
Beet western yellows, Curly top, Cucumber mosaic, Lettuce infectious yellows, and Lettuce mosaic Many viruses are difficult to distinguish from one another by symptom alone. In addition, for each particular disease, the symptoms will vary greatly depending on stage of plant at time of infection and other factors.
What to do
Buy and plant virus-free seed if available. Resistant varieties may be available.
Source
White Mold (Sclerotinia Lettuce Drop)
What you'll see
White mold is a distinctive disease that most often affects stems and foliage at the base of cole crops and lettuce plants. Affected tissue develops a soft, watery rot and white, cottony mycelium forms on the surface.
What to do
White mold is favored by a wet soil surface. Remove and destroy entire infected plants and crop residues as soon as you see the mold.
Source

Sources

Every figure above is backed by these.