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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.

The Effect of Light on Germination and Seedlings

Do seeds need light to germinate? And how does light affect the germination of seeds? T&M’s former Technical Manager, David Batty, investigates these questions and discusses the question of whether seedlings need light. Some plants germinate well in darkness, some prefer continuous light, and others have no preference either way.

Apparently it was custom in Ancient Egypt, before finally sealing the tomb, to leave a little pile of moistened corn near the sarcophagus. One can imagine the seed germinating in the pitch darkness, stretching itself upward feeling for light which was not there and finally toppling over having exhausted its food reserves.

It is a fact of life that most plants need light to grow and keep them healthy, but not all plants need light to germinate, and, as we shall see, some seeds find light a hindrance. If we look at the matter from the gardener’s point of view, however, we can use the rule of thumb that most cultivated plants on sale in seed form prefer to germinate in the dark. There are some notable exceptions however, some greenhouse perennials, epiphytes, many grasses, and even tobacco all prefer light and a large number of seeds are not fussy either way.

The reason is that commercially produced seed is bred and selected for its ease of germination as well as other more obvious characteristics and so peculiarities such as light or dark requirements do not often occur. On the other hand seed which is obtained non-commercially, in small quantities from the home gardener, seed lists, or the more unusual items from seed merchants may prove to be much more fussy in its requirements. In fact, research has shown that with seeds other than cultivated forms there is a great deal of variation. We can divide seeds of this type into those which germinate only in the dark, those which germinate only in continuous light, those which germinate after being given only a brief amount of light and those which germinate just as happily in light or darkness.

As long ago as 1926 experiments were carried out by Kinzel to find out the light requirements of hundreds of plant species. He found about 270 species which germinated at or above 20°C (60°F) in light, and 114 species germinated at the same temperature in the dark. He also found 190 species which germinate in light after experiencing hard frosts and 81 species likewise germinated in the dark. Fifty-two species germinated in the light and 32 species in the dark after light frosting and finally there were 33 species which were unaffected by light or dark.

Unfortunately, as with all gardening matters, things are not quite this simple. Other factors, it seems, can also affect the seed’s light requirements, for example, with some species (e.g. Salvia pratensisand Saxifraga caespitosa) light requirement only exists immediately after harvesting whereas with Salvia verticillata and Apium graveolens (Celery) this lasts for a year and to confuse matters further other species develop a light requirement while in storage. Chemicals also, such as nitrates in the soil, can substitute for light in stimulating seeds to germinate so that some light requiring seeds will still germinate if covered with fertile soil. Still it all makes for interesting gardening doesn’t it?

For a fairly comprehensive list of the Light/Dark requirements of seeds we refer you to Thompson & Morgan’s booklet ‘The Seed Sowing Guide’, which they will be pleased to send you for only 99p if you drop them a line. This is a helpful general guide but it is worth remembering that not all seeds in the same genus behave in the same way. For example Primula ohconia needs light and Primula spectablis needs darkness for germination, so there is still a lot to learn, much of which can only be gained by personal experience and sharing that information gained with others.

The explanation of how light affects some seeds and causes them to be in a state of readiness for germination and yet prevents other seeds if necessary from germinating is highly complex. Suffice it to say that it is mainly the light’s effect upon a plant pigment called phytochrome within the seed. This relates to the type of light which the seed receives. As a generalisation, light in the red wave length usually promotes germination whereas blue light inhibits it.

In a practical vein the light requirements of a seed may relate to the habitat in which the seed parent usually grows, so as to ensure that those which fall in an area conducive to growth will germinate and those which fall in less salubrious circumstances bide their time. For example a seed requiring light to germinate might fall into the deep shade of another plant where growing conditions would be very poor, whereas a seed falling into an open, well lit space would germinate quickly and flourish. On the other hand, it may be essential for the establishment of the young seedling that part or all the seed needs to be covered with soil or in the shade, perhaps, to protect the young root.

In such a case with a seed which required darkness, uncovered seed, which is exposed to light will not germinate. Sometimes only part of the seed is light sensitive. Phacelia is light sensitive at only two points on its surface and in a lettuce at only one. The micropyle where the water is absorbed, is light sensitive perhaps to ensure that only correctly oriented seed with the best chance of survival germinates.

Of course, the effect of light on seeds should not be over emphasised, no real hard and fast rules can be laid down, as other factors interact with light. To the gardener, the two questions he needs to have answered are ‘How deep should I sow my seed?’ and ‘Should I cover the seed tray to exclude light or not?’

In answer to the first question, depth of sowing depends a lot upon the size of the seed. Very tiny seed should normally be sown and left uncovered. Small seed which needs light will usually receive it even if you cover it with a light sprinkling of compost or vermiculite because light does travel a short distance through the soil and with some seeds exposure does not need to be long or continuous. For example tobacco seed receives all the light it needs to germinate, after it has taken up water, in 0.01 seconds of sunlight and even moonlight will do!

It is not just the very tiny seeds which sometimes need light to germinate, an average seed like Impatiens is light sensitive too and should be covered with a fine sprinkling of vermiculite after sowing and left in diffused light, placed in polythene to provide a high humidity until germination which usually takes 10-14 days at 21-42°C (70-75°F).

Medium sized seeds and upward, unless they have a light requirements (and we do not know of any really large seeds which do) should generally be sown just below the surface, enclosed in a polythene bag or cling film and placed in diffused light.

Some, but not all, popular seeds which prefer light for germination are:

  • Achillea
  • Alyssum
  • Antirrhinum
  • Begonia
  • Calceolaria
  • Coleus
  • Exacum
  • Ficus
  • Gaiilardia
  • Gerbera
  • Gloxinia
  • Helichrysiim
  • Kalanchoe
  • Nicotiana
  • Petunia
  • most Primula
  • Saintpauliu
  • Streptocarpus

Seeds which will only germinate in darkness should be sown at the correct depth and then covered in black plastic or similar to exclude all light until germination takes place. Cyclamen is a subject which should be treated in this way. Normally a difficult subject to germinate it proves far less so if sandwiched between moist filter paper and placed in a plastic container in total darkness. Usually germination occurs in about a month at 15-20degC (60-68degF) when the tiny corms can be transplanted into compost and grown on. The temperature, however, should be no higher than 20degC (68T) as high temperatures will induce a different form of dormancy!

Some other popular types which prefer darkness for germination are:

  • Calendula
  • Centaurea
  • Delphinium
  • Gazania
  • Nemesia
  • Primula sinensis
  • Schizanthus

Providing artificial light should not normally be necessary for seeds sown in greenhouses, well lit propagators etc. but if light is a problem or, more importantly, if you want to ensure rapid, healthy growth of your seedlings after germination then some form of additional light may be necessary. This would particularly be the case in raising seeds early in the season and quite a number of flower and vegetable seedlings respond to supplementary light. For example, tomatoes and cucumbers where vigour and earliness have been improved, also Antirrhinum, Stocks, Gerbera, Gloxinia and Gesnaria have all responded with a higher growth rate when given extra light in the winter months.

Tuberous begonias when sown in late winter must have supplementary lighting if they are to develop properly. They are sensitive to day length and when this is less than 12 hours they form tubers instead of making vegetative growth. In order, therefore to produce healthy young plants lighting must be given to extended the day lengths to more than 12 hours.

To provide this light, fluorescent tubes of the Gro-Lux type, to give light something akin to sunlight should be used, suspended around 2 feet (60cm) above the seedlings. As there will be so much moisture about use only approved horticultural fittings when installing the lights and fit a time clock if possible so that the lights can be on for 12 hours each day.

David Batty is a former Technical Manager at Thompson and Morgan Seeds, where he looked after the seed-testing laboratories.

Source of article
Growing From Seed – Spring 1989 Vol. 3 Number 2
Copyright: The Seed Raising Journal from Thompson & Morgan

Most plants need light to grow and keep them healthy, but not all plants need light to germinate, and, as we shall see, some seeds find light a hindrance.