You can sow lettuce indoors from early March. Regular sowings should provide fresh leaves until the end of October. If you are going to plant seedlings through a weed barrier, (cardboard or similar) you will need to sow some seeds in a container like this one which I saved after a supermarket shop. You need to make some drainage hole with the tip of a sharp knife.
You will first need to dig up enough soil from your garden to fill a small plastic container. Use a spade to dig up enough from a corner of your garden and remove any stones and weeds
Rub the soil in your hands to make it more friable.
Fill up your container to just below the top. Because garden soil can contain lots of weed seed sow them very thinly as in the photo in rows so you can identify them when they germinate and not weed them out by accident!
If you are unable to buy some seeds see if a neighbour or a friend have some spare. All you need is literally a pinch. Once you have sown the seeds sprinkle a little soil over them, gently firm and then, if you have a watering can with a rose, lightly water. Otherwise put the tray in a bowl of water for a few minutes to dampen the soil. You want it to be moist, not sodden. I like to add a label and date to remind me what I have sown.
Now you need to put the seed tray somewhere light and warm. A windowsill in the kitchen is ideal. If you have another clear plastic container the same size you can put it on top like a mini-cloche. Alternatively cover with some cling film.
In a week or so you will see the seedlings start to emerge. Keep the seedlings moist and grow on outside out of direct sunlight if you are keeping the cover on, otherwise a sunny spot is fine. Watch out for slugs nibbling them. It is worth checking the seedlings when it is dark and removing any slugs. But, hopefully this will not be a problem. As you can see in the photo, the seedlings are roughly in three rows. If any weeds emerge they will now be easy to identify and remove.
After two or three weeks the lettuce seedlings should have four true leaves and be ready for pricking out into a new container to grow on or be planted through your cardboard mulch spacing them 15cms x 15cms apart. Pricking out unused seedlings, spacing them about 5cms apart into a small container either using potting compost or garden soil, is a great way to make use of surplus plants which can be given to friends, family and neighbours .
When planting into their final growing position space your seedlings about 15cms apart in blocks or rows. As with all vegetables that are being transplanted lettuce can be planted through a weed suppressant like cardboard but if you have a patch of bare soil as I have here, you don’t need to use cardboard.
You should be able to pick tender outer leaves in about a month and whole lettuce can be pulled after about 6 weeks. Having transplanted your lettuce seedlings sow some more for a continual supply.