When To Transplant Lettuce Seedlings Outside

If you have a greenhouse, polytunnel, or indoor grow light setup, you’re already luckier than most gardeners. This means you can cheat the cold weather and start seeds under cover where they can comfortably germinate. Lettuce is one of those favorite vegetables that we’re all excited to grow come spring.

But if you’re new to gardening, you might have a lot of questions about growing lettuce. Should you sow it directly in the ground or try starting it in trays? Does lettuce transplant well? When and how to transplant it? We’ll do our best to answer these questions (and many more), to help you give lettuce a head start.

You should transplant lettuce outside after it has grown undercover for 4-6 weeks, and reached the height of 2-3 inches. The earliest time to do this is 3-4 weeks before the last frost, but you can transplant lettuce any time after that. Allow for 7-10 days of hardening off before moving outside.

Why transplant lettuce seedlings?

Growing lettuce directly in the ground is easy, especially when setting it up for the “cut-and-come-again” method. You just sprinkle the seeds in rows and you’re good to go. Often times you don’t even need to thin loose-leaf lettuce, since the stronger seedlings tend to suppress the smaller seedlings from growing.

So why go through all the hassle of tending to lettuce seedlings for weeks to come? The answer is simple: growing head lettuce requires different conditions than growing loose-leaf lettuce.

By starting head lettuce seedlings indoors or undercover you will:

• Get an early start. You’ll be able to transplant your lettuce seedlings at an advanced stage of growth – while the lettuce sown outside will be barely emerging from the ground. An early crop of lettuce leaves sounds fantastic during the hungry gap.
• Get the ideal spacing. Head lettuce varieties need room to grow and expand. With direct sowing, you often need to do some extra sowing, followed by thinning to get the exact positioning you want. With transplanting, you just pop the lettuce seedlings in the ground 10 inches apart and you’re good to go.
• Avoid bald spots. Whether we’re dealing with old seeds, waterlogged soil, slugs or other nuisances, we all have losses with our seedlings. Some seeds just don’t germinate. Others germinate only to get eaten or dampen off. That’s how we end up with empty patches in our gardens. By transplanting lettuce seedlings, and keeping some spare seedlings always on hand, we can easily fill up these spots in the garden.
• Avoid extra work. Transplanting lettuce seedlings gives you the opportunity to prepare the soil and clear it of weeds beforehand. This will make plant care and maintenance and easy task. There’s nothing more annoying than trying to weed rows where lettuce has barely germinated.

When are lettuce seedlings ready to go outside?

You can transplant lettuce seedlings at almost any stage – even when they’re really tiny and have no true leaves yet. But, to ensure excellent success rates, it’s best to transplant lettuce seedlings when they’re a bit older – at least one month, if not 6 weeks from germinating.

So transplanting shouldn’t be done according to what date’s on the calendar, but rather, what growth stage your lettuce seedlings are in.

Sowing head lettuce

You can start sowing lettuce for growing outside about 7-8 weeks before the last frost date. Lettuce is frost-resistant, but even so, you don’t want to hurry and move it outside too soon. Temperature is what dictates lettuce’s rate of growth, so if it’s really cold, it won’t grow much no matter how eager you are.

When sowing lettuce, keep in mind that we’re going to prick it (more on that in a minute). So the container where you first sow your lettuce will only host it for about a week. This means you can grow a LOT of lettuce (hundreds of tiny plants) in the space of a small Tupperware.

Sow the varieties you want in rows (or separate bowls) and place markers so you don’t mix them up. Sprinkle with soil to cover the seeds loosely, lightly mist the surface and you’re good to go. From now on to germination, you’ll only need to mist the container every 24 hrs.

Pricking lettuce

Pricking lettuce is the act of removing one individual seedling and placing it in a new growing medium – a cell inside a module tray, a seedling block, or a small pot.

You’ll find plenty of resources on how to prick seedlings (like my tutorial here), but there are only a few things you really need to know:

• Never allow lettuce to grow past its first true leaves before moving to a larger container. Ideally, you’ll want to prick lettuce seedlings right after they’ve germinated, in 7-10 days.
• Tease the seedlings apart with a pencil and remove them one at a time, holding them by their leaves. Try not to touch or squeeze the stem.
• You’ll get a long, spindly-looking stem and root, and that’s ok. Using the same pencil, make a hole in the cell/soil block, and bury the root & stem, guiding the seedling with the tip of the pencil. Cover with soil up to the leaves.

Congratulations, you’ve just learned how to prick and transplant lettuce into module trays! Now you just have to repeat the process over and over again. If you find pricking tedious, that’s because it is – it’s quite annoying but there’s really no way around it if you want sturdy lettuce seedlings.

Growing lettuce in module trays

Now that you’ve pricked the number of lettuce seedlings you want to grow (plus a few extras), it’s time to nurture them until they’re ready to go outside. I use this type of module trays to fit 72 lettuce seedlings – enough to last me throughout spring and summer.

Since they won’t be crowded in their little individual cells, lettuce seedlings will start to grow true leaves and soon resemble real mini-lettuce plants.

At this stage, you’ll want to water them from below, ensure proper ventilation and make sure that they get 16 hrs of light if you’re growing them under grow lights. Regular watering is essential, and you can easily tell if lettuce seedlings are underwatered because they’ll start to look droopy. But if you catch this in time, they’ll recover.

After about 4 weeks, you’ll start to see root balls forming, which means your lettuce seedlings have taken up all the available growing space inside the cell. You can still delay transplanting for a maximum of 2 weeks at this point, but you might need to fertilize them with a water-soluble fertilizer.

Hardening off lettuce seedlings

Hardening off seedlings is an important step that many gardeners are tempted to skip. Lettuce is not an exception, even if it’s frost-resistant, it still needs to be hardened off.

When the lettuce seedlings are 4-5 weeks old, begin to take them out for 1-2 hrs at a time, in the shade, gradually increasing the time you leave them outside, as well as the sun exposure. After spending a couple of nights outside, lettuce seedlings are ready to be transplanted into the ground.

If you skip this step, you might expose your plants to transplant shock, and while you won’t lose them altogether, it will significantly stunt their growth.

Another way to protect your young seedlings from frosty nights and hungry birds is by adding a layer of horticultural fleece on top of the raised bed until the danger of frost has passed.

What to do about leggy lettuce seedlings?

If you’re dealing with leggy seedlings, you might have made one of the two mistakes:

A. You’ve sown them in a container and didn’t prick them out. Lettuce seedlings tend to compete for light, so they will grow tall to reach it. That’s why pricking is so important to place them back at soil level.

B. You’re growing your lettuce seedlings on a windowsill, your lights aren’t strong enough or you’re dealing with cloudy weather in your greenhouse. You can prevent this from happening by adding more light (or getting better lights – here’s the model I use and love) or increasing the duration of light exposure to at least 16 hrs.

Leggy seedlings won’t necessarily recover, but you can plant them a little deeper when transplanting them outside.

Transplanting lettuce outside – how deep?

When transplanting lettuce seedlings in their final position, whether that’s a raised bed or a container, you’ll want to plug holes with a dibber to mark where the seedlings will go. The holes don’t need to be very deep (about the depth of your cell trays), as it’s best to try to transplant lettuce seedlings at the same soil level. But, if you end up plugging them a little deeper, there’s no harm in that either – they will grow up.

Alternatively, you can use a hand trowel for this task, but I’ve found using a dibber a lot more practical for transplanting a large number of seedlings.

Space the head lettuce seedlings at least 10 inches apart to allow for their head to form to full size. You can then harvest them as a full head of lettuce, or continuously pick the lower leaves for months to come, as the plant regenerates.

Conclusion

I like to always have lettuce seedlings on hand to ensure a continuous supply of greens in my garden. The only time it’s too hot for lettuce outside is the end of June – the middle of August, so judging by this interval, I like to start lettuce seedlings both in early spring and late summer.

Starting lettuce in trays requires a bit of babysitting, but transplanting them outside is a breeze – that’s the last time you’ll ever have to work on them. So choose your varieties wisely, and good luck with your lettuce crop!