Lettuce and light

Jun 29, 2022 | Gardening tips

Light-dark requirements for lettuce seed germination

Top tips:

  • lettuce seed needs light to germinate
  • lettuce seed has a shelf life of one year
  • prepare a firm, moist seed bed for planting lettuce and keep seed moist at all times
  • know the light and dark requirements for each seed before planting

You may be thinking of sowing lettuce given their current high price in the shops so here are some tips for success as well as some information on the light and dark requirements of various seeds.

Lettuce is incredibly easy to grow and you will be eating it in about 7 weeks, but it has 2 specific requirements. Firstly, lettuce requires light to germinate its seed so do not cover punnets with soil. Secondly, lettuce has a shelf life of about one year. It will germinate beyond that, especially if well stored, but the viability will be reduced, and will reduce each year following. Starting seed in punnets and keeping them in a warm place is best right now in the midst of winter as the seed germinates poorly below 10C. Transplant into beds when a good size. Click the link below to read a previous more extensive article by me called ‘Growing salad greens’.

Seeds that require light to germinate:

Some seeds require light to germinate, some darkness and some are not fussy. Those that require light, if covered with soil, will remain dormant. Not treating seed correctly in regard to its light-dark requirements is often the reason for seed failure. Some seeds need only a very light covering of soil to germinate while others need total darkness.

Vegetables that require light include lettuce and celery and the herbs dill and savory. These are all very small seeds and can dry out very easily. Prepare a punnet or seed tray with seedling mix (not potting mix as the tiny seeds will fall through) and water well, and then firm the bed down. Onto the moist bed, sprinkle seed and press it down so that it makes contact with the seedling mix but is still visible. Unless this happens, the seed will not root into the mix. Keeping the seed moist is essential as tiny seeds dry out and die. Each day check that the bed is moist and if not, use a spray bottle with a fine jet.

The more common flowers that require light to germinate include achillea, begonia, coleus, columbine, geranium, impatiens, lobelia, petunia, poppies, some salvias and snapdragons.

If you need to direct-sow light germinating seed into garden beds, you will need to protect it from washing away or blowing away or being eaten by birds or vermin. Covering the seed with plastic or cling film attached to a wire basket for instance, works well, as long as it allows sun through unimpeded. However you may need to pay extra attention to watering as the covering may speed up the drying of the soil and seed. Very fine ground horticultural vermiculite can be used as a covering to prevent this but it is not the vermiculite commonly available from hardware stores or from the community garden. It is fine enough to allow light penetration and will help retain moisture required to keep the seed alive.

Seeds that require very shallow soil

Seeds that require very shallow soil coverage include the common members of the brassica family, kale and kohlrabi (also brassicas), and leeks.

Seeds that are not fussy

Seeds that are not fussy include tomato, eggplant and peppers which are all members of the Solanaceae family, and cucumber, melons, squash and pumpkin, all members of the Cucurbitaceae family.

Common flowers include alyssum, aster, cosmos, dianthus and marigold.

Seeds that require the dark

Seeds that require the dark (pitch black is best so cover with black plastic) and a covering of soil 3 times their diameter include onions, nasturtiums, calendula and sunflower.

Remember that although seeds require varying amounts of light to germinate, all seedlings require light to grow.

Growing salad greens

Written by Robin Gale-Baker