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Do Seedlings Need Light Before They Sprout?

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Seeds, when they come to us in seed packages, are dormant. For a plant to grow from the seed, that dormancy has to be broken. Some seeds break dormancy when their seeds get wet. Some require that fire or pressure break their husk. Others require light.

Light Versus Dark

The light requirements for seeds vary widely. Some seeds need light to germinate. Other seeds will germinate only in darkness. Still others seem to have no preference and will germinate in either light or darkness if their temperature and moisture requirements are met. Some seeds — meadow sage (Salvia pratensis) and tufted saxifrage (Saxifraga caespitosa), for example — require light to germinate if you attempt to grow the seeds immediately after harvesting them. If you hold the seeds over the winter, however, they loose the light requirement and will germinate in darkness.

The Mechanism

A seed’s need for light is tied to a pigment called phytochrome, which exists inside most plants. Photochrome reacts chemically to light. This pigment is involved in plants growing upward to seek light. It is what determines that plants that grow in low light get longer and more spindly than plants grown in enough light. When light hits light-sensitive seeds, the photochrome inside the seeds reacts, causing the seed to break dormancy and sprout.

Seeds that Need Light

According to Norman C. Deno, author of “Seed Germination: Theory and Practice,” swamp plants typically need light to germinate. Woodland seeds also often need light as do many weed seeds. That’s why when you cultivate to get rid of weeds, others pop up as soon as they receive enough light to grow. Vegetable seeds that need light to germinate include dill (Anethum graveolens), lettuce (Lactuca sativa) and some peppers (Capsicum annuum). Coleus (Solenostemon scutellarioides), snapdragon (Antirrhinum majus) and Shasta daisy (Leucanthemum Leucanthemum x superbum) are among the flower varieties that need light to germinate.

Planting Seeds That Need Light

The general rule of thumb is to plant a seed no deeper than its length. Seeds that need light to germinate must be planted shallower than this, however. One method is to sow the seeds over the surface of the soil. Using your hand or the back of a trowel, gently press the seeds into the soil so the tops of the seeds remain uncovered. If the seeds are tiny enough that they are likely to blow away, you can sow them on the surface of the soil, sift a fine layer of compost over the top and water the compost. Doing so will hold the seeds in place while allowing light to penetrate.

Do Seedlings Need Light Before They Sprout?. Seeds, when they come to us in seed packages, are dormant. For a plant to grow from the seed, that dormancy has to be broken. Some seeds break dormancy when their seeds get wet. Some require that fire or pressure break their husk. Others require light.

Seeds that Need Light for Good Germination

Starting seeds is not complicated, but you do need to know what conditions the seeds you’re planting need. Many gardeners are unaware that some seeds require light to germinate and covering them with soil will inhibit their sprouting.

There’s a general seed planting rule that says you should plant a seed to a depth that is three times its thickness. That means fat bean seeds should be planted one to three inches deep and tiny carrot seeds should barely be covered. Most seed packets will take the guesswork out of the process and tell you how deep to plant the seeds. It’s a good idea to follow these recommendations because a seed that is planted too deeply might not have enough stored energy to push itself above the soil line.

There’s an exception to every rule, though. Some seeds need the stimulus of light hitting them before they will break dormancy and start to germinate. Very often it is seeds that self-sow that require light. These plants, such as balloon flower (Platycodon grandiflorus) and poppies, drop their seeds on the soil and they germinate where they land. They sprout in response to environmental factors, including having the light hit them.

Seeds That Need Light to Germinate

There are several seeds that germinate best when they are exposed to light. If these seeds are covered in soil, chances are they will remain dormant and not sprout until conditions improve.   It seems counterintuitive not to bury seeds, but these seeds should only be pressed onto the surface of the soil and kept moist to germinate. They include:

  • Ageratum
  • Balloon flower
  • Begonia
  • Browallia
  • Coleus
  • Columbine
  • Gaillardia
  • Geranium
  • Impatiens
  • Lettuce
  • Lobelia
  • Nicotiana
  • Osteospermum
  • Petunias
  • Poppies
  • Savory
  • Snapdragons

Seeds That Will Germinate With or Without Exposure to Light

While most plants that self-sow in your garden are able to germinate without being covered with soil, that doesn’t necessarily mean they absolutely need light. Some plant seeds are indifferent to light exposure and simply need to make contact with soil, whether it is underneath them or covering them. Flowers such as alyssum and cosmos will self-seed during their current growing season as well as the next one, whether or not they are exposed to light.   Other seeds that will germinate uncovered include:

  • Alyssum
  • Aster
  • Balsam
  • Cole Crops
  • Celosia
  • Cosmos
  • Cucumbers
  • Dianthus
  • Eggplant
  • Marigold
  • Melons
  • Pepper
  • Squash
  • Tomato
  • Zinnia

Although the seeds listed above do not require a covering a soil, you will probably get better germination if you follow the recommended planting depth because it will be easier for you to keep them moist and safe from hungry birds.

Keeping Seeds Moist When Exposed to Sunlight

Being able to sow seeds on the surface of soil makes planting easier, but keeping them moist until germination can be difficult since they are exposed to more than just light. Animals, wind, heavy rain, and digging gardeners can all disturb or remove seeds from your garden. If you are growing your seeds in flats or containers, you can cover them lightly with plastic wrap, plastic domes, or tuck them inside of clear plastic bags. They will still be exposed to sunlight, but they will not dry out as quickly as if they were left open to the elements.  

For seeds directly sown outdoors, another option is to cover the seeds with a thin layer of fine vermiculite, a naturally occurring mineral with water-holding properties. Vermiculite is porous enough to let the light shine through while retaining enough water to stay in place and keep the seeds and soil under it moist. It can usually be found near seed-starting supplies; look for finely ground horticultural vermiculite, as other types are not suitable for gardening.

This may all sound complicated, but most seed packets will tell you exactly what you need to know. And remember, seeds have been sprouting for millennia without a lot of fuss––it’s just nice to give them the best chance possible by providing them optimal conditions so they can thrive.

Some plant seeds need exposure to light to germinate and should not be covered with soil. Here are some tips for starting seeds that need light.