Lettuces are a year-round salad crop well-suited to British gardens. They are valued mainly for their leaves (though varieties grown for their stems are apparently popular in China) and do best in a moist but well-drained soil that is fertile and neither too acidic or too basic. They are also perfect for container growing, being ideally-suited to growbags as well as compost-filled pots, troughs and trays.
Managing seeds and germination
Lettuce seed are quite small, with about 1000 seeds per gram. Given their size, they should be sown quite shallowly – about 3 to 6mm deep is about right.
The optimum temperature for germination is 18 to 21ºC, though the seed will germinate well below the optimum. Whatever you do, avoid temperatures of 25º C and over; otherwise, germination might be inhibited by thermodormancy (see section on ‘Problems’ below).
Sowing, growing and harvesting strategies
Sowings for outdoor crops can be made from late winter to summer. Lettuces are a very flexible crop, and there is no one way of growing or harvesting them. Probably the most common way, is to grow individual plants allowed to grow to full size, but there are several other ways:
- Single harvest of full-sized heads
- Cut-and-grow-again for teenage leaves
- Cut-and-grow-again for baby leaves
- Easy Peasy Gardening for baby leaves
The instructions we give below are quite detailed and may be difficult to follow on first reading. Do not, however, be intimidated –there is a lot of flexibility in growing lettuces, and the instructions can be adapted to fit your needs.
1. Single harvest of full-sized heads
In the traditional way of managing lettuces plants are grown at wide spacings and harvested once when they are full-sized. All lettuce types are suitable for this one-time harvesting, and they can be grown either in the well-worked ground of your garden or in large, compost-filled containers such as trays, troughs and grow bags.
For the best results, the plants must not be crowded, so leave plenty of room around each one. One way to do this is to space the plants equidistant from each other, preferably in a triangular pattern. As a general rule of thumb, leave about 30cm each way, reducing this distance to about 20cm for small varieties such as Tom Thumb and Little Gem.
Starting the plants
There are two basic sowing strategies that can be employed to start the plants:
- Direct seeding
- Starting as transplants
In this method, the seed is sown directly into either compost-filled containers or the ground, where the plants are left to grow until they are harvested. To start the plants, sow two or three seeds in each ‘post’. After germination, thin the seedlings down to leave one per post, and let the plants grow and develop.
Starting as transplants:
This is a two-stage process that involves sowing the seed into a module tray, then transplanting the young plants into their growing place.
For the first stage, sow two or three seeds in each cell of a compost-filled module tray. After germination, thin the seedlings down to leave one in each cell, keep well-watered and give a high-nitrogen liquid feed two or three weeks after sowing. For the second stage, transplant the young plants to their final spot when their roots fill the cells of the tray.
There are three main sowing times:
For early crops: Sow undercover from mid-February to mid-March; transplant outdoors when the plants are big enough and the weather warms up. Direct seeded containers can be moved outside when weather warms up.
For main crops: Sow either undercover or outdoors from late March to late July. Due to thermodormancy avoid sowing undercover in hot weather, and even outdoor sowings may germinate better if kept in the shade. Direct seeding can be done into the ground.
For late crops: Sow outdoors between early to mid-August, in containers or in the ground. Due to thermodormancy sowings may germinate better if kept in the shade if the weather is hot. Because the weather is likely to become cold and damp as autumn approaches, use mildew resistant varieties, preferably those that mature early. You could also put the crop undercover when the weather turns; those in containers can be moved inside while those in the ground can be covered with a cloche or coldframe. Otherwise be prepared to harvest the plants before they are fully mature.
Growing lettuces on
Throughout the life of the crop, make sure that that the plants are watered well enough to prevent a check in growth. Pay particular attention to those grown in containers – the compost tends to dry out quite quickly, especially on hot, sunny days. If the containers are not too big they can be placed in shallow trays filled with water – this maintains a reservoir of moisture that can last for days.
For lettuces grown in the ground, work a complete solid fertiliser into the soil before sowing or transplanting. Done right, there will be no need for extra feeding throughout the lifetime of the plants. This is not true, unfortunately, for a compost-grown crop, which will need to be fertilised with a liquid feed 2 to 3 weeks after sowing or transplanting. This may provide enough nutrients to see the crop through, but keep an eye on later-maturing varieties as they may need an extra application.
Plants grown in the ground will need to be weeded regularly. Be sure to keep you hands tools and destroyed weeds near the ground as if soil is dropped into the lettuce leaves it will spoil the eating later.
To harvest, cut the plants at ground level, just below the lowest leaves. Heading types should be harvested when their heads are firm and fully-formed. ‘Firm’ is a relative term, and while mature crisphead and semi-cos types are quite solid to the touch, the butterheads and taller cos types tend to be relatively soft.
Because loose-leaved lettuces, e.g. Red Salad Bowl and New Red Fire, don’t form a head, the optimum harvest time is not as clear cut as it is for the heading types. The plants continue to grow in size until they bolt, so for the biggest yields wait until the plants have filled out with leaves. Each variety behaves differently, and experience will tell you when it is best to harvest. Leafy lettuces are also well-suited to cut-and-grow-again harvest systems (see below).
Most gardeners will waste some plants as they bolt before all the lettuces are harvested. To avoid this happening it is worth starting harvests before the plants have reached full size. With a single lettuce harvested each day it won’t be long before they reach full size.
Achieving a regular supply
Unharvested lettuces will eventually bolt, which signals the end of their productive life. Because of this inevitability, if you want a continuous supply throughout the growing season crops must be sown at regular intervals. Initially, try sowing a limited number of plants every two to three weeks until you get a feel for the varieties you’re growing – they exhibit considerable differences in growth rate and bolt resistance. Then factor in the speed at which you eat the crop, while making seasonal adjustments to accommodate the fact that lettuces grow faster when the weather is warmer. Eventually, you’ll learn how many plants to grow at each sowing, and how frequently to sow in order to control gluts and shortages.
2. Cut-and-grow-again for teenage leaves
Though growing lettuces for a single harvest when the plants are full-sized is the most common approach, lettuces can also be harvested under a ‘cut-and-grow-again’ harvest regime. Under this system leaves are harvested two or more times from a single sowing. The plants are usually grown closer together, and harvests are made quite quickly after sowing – normally before the leaves reach full size.
Cut-and-grow-again harvesting is very flexible, and the leaves can be harvested in a range of sizes. At one extreme are the baby leaves, which are, as the name implies, quite small – we recommend harvesting when the plants are 10 to 15cm tall (see below).
Further up the scale are ‘teenage leaves’, harvested when the plants are larger than 15cm. Leafy varieties are ideal for the production of teenage leaves, and the growing technique is essentially the same as for full-size lettuces. The only difference is the spacing between plants is closer.
Grow plants 10 to 25 cm apart, spacing them equidistant from each other, preferably in a triangular pattern. Go for wider spacings if you plan on harvesting bigger leaves; leave less space between plants for harvesting smaller ones.
Leaves are harvested when the plants are anywhere between 15cm tall and full size. The larger outside leaves can be cut off the plants, leaving the inside ones to continue growing for later harvests. Alternatively, the whole plant can be harvested at one go, leaving a 5 to 6cm stump to regrow.
3. Cut-and-grow-again for baby leaves
This is a variation of the cut-and-grow-again system in which plants are grown at high densities, and harvested as ‘baby’ leaves. Loose-leafed types, such as the Salad Bowl varieties, are especially good for baby leaf production, though also worth considering are the cos types, such as Loboits and Little Gem, which don’t form heads when grown close together.
Growing lettuces for baby leaves is quite straight forward since transplants are never used. Instead, the seed are scattered directly where the plants are to grow and develop. Large, compost-filled containers such as grow bags and troughs are ideal for this system. Seed can also be sown in the ground, but weeding is difficult as the plants are so close do choose a site where the soil is relatively weed-free.
To get started, the seed can be scattered over the area, or sown into rows, about 1cm apart in rows 10 to 12cm from each other. After they have emerged, thin out the seedlings, leaving 2 to 3cm between them. To harvest, cut the leaves when they are 10 to 15cm tall, leaving a 1 to 2cm stump – the plants will regrow to give at least one more cut.
There are three main sowing times for outdoor crops:
For early crops: If you are using containers, sow them undercover from mid-February to mid-March – they are then put outdoors when the seedlings are established and the weather warms up.
For main crops: Sow seeds outdoors from late March to late July.
For late crops: Sow outdoors from early to mid-August in containers or directly into the ground. Because the weather is likely to become cold and damp as autumn approaches, you could put the crop undercover to extend harvesting; those in containers can be moved inside while those in the ground can be covered with a cloche or coldframe.
Whatever you do, don’t the let plants dry out: otherwise there will be a check in growth. This is a particular problem with container-grown plants, so in dry spells, be sure to water the plants thoroughly. One way to do this is to place the containers in shallow trays filled with water – this maintains a reservoir of moisture that will last for days.
Add a complete solid fertiliser to the soil before sowing in the garden – this may provide all the fertility the crop needs during its lifetime. For container-grown crops, periodically give a feed of liquid fertiliser, starting two or three weeks after sowing.
4. Easy Peasy Gardening for baby leaves
Easy Peasy Gardening is an alternative cut-and-grow-again system used to produce baby leaves, although in this instance the seeds are broadcast in compost-filled trays, troughs and pots. Because it is simple to set up and run, this system is ideal for children and time-stressed adults. All you have to do is the following:
- Use both leafy and cos lettuces. Both are excellent, and the cos types won’t form heads when grown close together.
- For early crops, sow seed under cover from mid-February to mid-March, using compost-filled troughs or pots – these are then put outdoors when the seedlings are established and the weather warms up. From late March to mid-August, sowings are made outside, though the containers may need to be put under cover if the weather turns cold. On hot sunny days newly-sown containers can be kept in the shade to avoid poor germination from thermodormancy.
- Broadcast the seed quite densely.
- After they emerge, thin out the seedlings to leave about 2 to 4cm between each one.
- Harvest when the plants are 10 to 15cm tall, leaving a 1 to 2cm stump to regrow.
Keep the compost moist at all times. One way to do this is by placing the containers in shallow trays filled with water.
Feed the plants with liquid fertiliser about two or three weeks after sowing. Fertilise again at least once after the first harvest, though it is difficult to say when exactly is the best time: this depends on the growth rate of the plants, intensity of the rainfall and frequency of watering. For more information about Easy Peasy Gardening click here.
Thermodormancy in lettuces
The optimum temperature for lettuce seed germination is between 18 to 21˚C. Research conducted back in the 1970s showed that germination may be inhibited at temperatures above 25º C due to a phenomenon called thermodormancy. This inhibition, however, was not universal: crisphead varieties were found to be quite resistant, while butterhead types were more susceptible. Soil and compost temperatures can easily surpass 25º C in late spring and summer, especially in greenhouses and tunnels. This means it is better to sow outdoors during periods of hot, sunny weather. Even with outdoor sowings thermodormancy can be an issue, and anything that keeps the temperatures down during germination will reduce the chances of poor germination. Even though lettuces display different levels of resistance, it is probably safer to treat all varieties alike just to be on the safe side. Some of the strategies you can employ to reduce thermodormancy include the following:
- Sow early in the afternoon, somewhere between 2:00 and 4:00pm. Sowing at this time of the day means that the temperature-sensitive stage of germination (yes, there is one) can occur at night, when it is cooler.
- When the weather is hot, put trays, pots and grow bags in the shade until the seedlings emerge, or you can shade that part of your garden where the lettuce seeds are sown.
- Prompted by high temperatures and long days, all lettuces will eventually bolt, that is go to seed, for more information click here . Lettuces vary enormously in their resistance to bolting, and, though it can’t be avoided, it can be delayed by growing bolt-resistant varieties.
Slugs and snails
These are pernicious pests that can ruin a crop of lettuces, especially at the seedling stage of development. They are effectively controlled by nightly foraging trips to the garden – just collect them up and drop them into a bucket containing a bit of salt. Also effective are predatory nematodes and the application of slug pellets, some of which are safer than others so read the label before applying.
Rabbits and deer
Other than shooting the critters, the only way to prevent damage by these animals is to cover the plants with either environmesh, tendamesh or fleece.
These are tiny sap sucking insects that attach themselves to both the leaves and roots of lettuces. A simple way to control these pests is to cover the crop with fleece or tendamesh.
Characterised by brown patches and fuzzy growth on the leaves, this fungal disease can be a problem when the weather turns cool and moist. Many lettuces have some resistance – though the degree of resistance varies from variety to variety.
For more information see the RHS website.
© Michael Michaud