Some Seeds Need Light to Germinate

By Ken Lain, the mountain gardener

When I was a young man a dear friend taught me to plant seed to a depth that is three times its thickness.  That means that chubby bean seeds or peas should be planted 1-3 inches deep, while tiny carrot seeds need only be barely covered with soil.  It’s a good idea to follow these recommendations, because a seed planted too deeply may not have the energy stored within it to push through the soil and into the light.

Seed packetsHundreds of spring seed packets have arrived at the garden center, many labeled organic or non-GMO.  Most of the packets also carry very good instructions for exactly how deep to plant each seed.

The amount of light needed for each seed to germinate, however, is scarcely noted on the packets.  Some seeds actually germinate best when exposed to light; if these seeds are covered with soil, chances are that they will never sprout. To ensure germination, these seeds should be pressed lightly onto the surface of the soil and kept moist.

Seeds that germinate best when exposed to light:

Ageratum       Balloon Flower          Begonia          Browallia        Coleus             Columbine       Geranium                   Impatiens       Lettuce           Lobelia Nicotian Osteospermum Petunias                 Poppies          Savory            Snapdragons

Seeds of most self-sowing plants can germinate without being covered with soil. However, that doesn’t necessarily mean that they need light. Some plant seeds are indifferent to light exposure.

Seeds that don’t need light to germinate:

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

How to Keep Light-Exposed Seeds Moist

Being able to sow seeds on top of garden soils makes planting easier, but keeping them moist while germinating can be difficult.  Although these seeds don’t require a covering of soil, the germination increases if you follow the recommended planting depth, simply because it’s easier to keep them moist and safe from hungry birds.

Most local gardeners start their seed in flats or containers that they cover with a light plastic wrap, plastic domes or, even tuck them inside plastic zip closure bags.

Organic gardeners prefer not to use plastics around their plants.   They prefer to cover seeds with a thin layer of fine vermiculite. Vermiculite is porous enough to let light shine through, while retaining enough water to stay in place and keep the seeds and soil under it moist.

Watters carries commercial size (4 cubic feet) bags of vermiculite, and smaller sized bags that we keep near the seed-starting supplies.  Vermiculite flecks are often incorporated in our seedling mixes; these are the small silvery metallic flecks that shine in a good potting soil. The particles soak up water and nutrients and hold them in the mix until the plants are ready to use them.

If making your own soil mixes, be sure to use finely ground horticulture vermiculite. The vermiculite sold in home improvement stores is used in insulation and plaster and does not have the same water absorbing quality.  This is an example of when it pays to use a garden center for advice.