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:
- most Primula
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:
- Primula sinensis
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.
Welcome to Larry Hodgson’s world
Is Light Necessary for Germination?
Question: I have a question regarding the germination phase of sowing seeds. Is light absolutely necessary or will heat and humidity be enough to get seeds to germinate? Then, when the stems begin to emerge from the soil, I assume it is time to supply light until I can acclimatize the young plants and transplant them into the garden. Did I understand that right?
Answer: You pretty much have it, but… I still recommend exposing the seeds to light as soon as you sow them. Here’s why.
First, true enough, light isn’t necessary to germinate most seeds: the majority are perfectly capable of germinating in complete darkness. But there is a significant proportion of seeds that will not germinate if they aren’t exposed to light. This includes maybe a third of the plants we commonly grow from seed.
Seeds That Need Light to Germinate
Among the well-known plants whose seeds need light to germinate or, at least, for which exposure to light stimulates better germination, are the following:
- Primrose (most species)
- Sweet alyssum
In general, seeds that need light to germinate, like these nicotiana seeds, are the very smallest ones. Photo: Mostlly Tim, flickr.com
Note that most seeds that need light to germinate are very small, sometimes almost dustlike. On the back of the seed pack, it often recommends not covering them with soil, but simply pressing them into moist soil when you sow them. Now you know why! It’s because all but a very thin covering of soil would place them in the dark!
Probably two-thirds of the seeds you buy are of varieties that are light indifferent. Photo: gardenerspath.com
There is no need to list these, as they make up the majority of seeds. (Hint: they’re usually seeds of medium to large size.) Almost all vegetables, except lettuce, fall into this category. Herbs and annuals, though, are more of a mixed bag.
These seeds will germinate with or without exposure to light. So, logically, you could place them in a place with little or no light at first, then expose them to light only when they germinate. But they’ll germinate just as well if exposed to light.
Personally, I expose light-indifferent seeds to light as soon as I sow them. This is partly pure laziness: it’s just one step less to think about. And also, it allows me to have all my seedlings in one place rather than having two locations. Plus, this way, there is no problem if you have to be away for a few days and are not present to move the trays after germination (you shouldn’t leave sprouted seedlings in the dark more than a day) … or if you’re the forgetful type and don’t check your seeds daily.
Seeds That Need Darkness to Germinate
These are the oddballs of the seed-sowing world. I mean, it just doesn’t seem logical that a seed would need darkness to germinate, does it, not when all green plants need light to grow? And there aren’t many of them, but they do exist. Most are forest species, not often grown in home gardens.
So, how should you handle these eccentrics? There are essentially two possibilities.
The first is the most obvious: simply make sure that you cover these seeds with soil at sowing time. That will keep the light off even when the seed tray is exposed to light. For medium and large seeds, therefore, you don’t have to do anything special. Just sow them at the recommended depth, usually 3 times the height of the seed, and no light will reach them.
Nasturtium seeds require deep sowing, putting them out of reach of sunlight. Photo: Hans Braxmeier, pixabay.com
Nasturtium seeds, for example, are huge (for seeds, that is!), nearly the size of beans, and their recommended planting depth is ¾ inches (2 cm), certainly deep enough that no light will reach them. You can therefore immediately place a pot of nasturtium seeds in a lighted spot with all your other seedlings.
You can insert trays of seeds needing darkness into a black trash bag until germination. Photo: http://www.destinationevents.ne
For seeds that are surface-sown or where the recommendation is to “barely cover the seeds with soil,” like the tiny seeds of delphiniums and coriandre, you really will have to place the seed tray in the dark, perhaps in a closet or inside a black trash bag or cover it with a wooden board or a sheet of something opaque to cut off any light.
Note that, even if darkness is necessary for the germination of these light-intolerant seeds, as soon as they have germinated, that is to say as soon as you see small pale sprouts appear, you do have to immediately move them to a well-lit location. So, check them daily.
Here are some examples of seeds that need darkness to germinate. Those indicated by an asterisk are surface-sown or barely covered in soil and so need to be placed in the dark. The deeper soil covering of the others means no light will reach them anyway and you could place their trays in a lighted spot for germination.
- Bachelor’s button
- Madagscar periwinkle or vinca*
- Schizanthus or butterfly flower*
- Sweet pea
- Verbena, hybrid*
Keep Light Moderate at First
An east or north window is best for freshly sown seeds. Temperatures could soar if the plastic-covered trays were immediately placed in full sun. Photo: girottifamily.typepad.com
If you decide to place your trays of freshly sown seeds in the light, you should still avoid full sun at this stage. Usually, at the beginning of germination, you will have covered your seeds trays with a transparent dome, a sheet of glass or plastic or a clear plastic bag. The goal is to maintain high humidity and stable temperatures, important at germination for almost all seeds. (Even cactus seeds germinate best under high humidity!) But a covered tray can readily overheat if exposed to full sun. The temperature inside this sort of mini-greenhouse can easily reach 140 °F (60 °C) or more, fatal for most seedlings.
You therefore need to put seed trays where they receive moderate light, such as a window facing east or north, or a spot under a grow lamp that gives off little heat, such as fluorescent or LED lights.
Temperature During Germination
The ideal temperature for germination of most seeds is between 70 and 75˚F (21 and 24 ̊C). It’s easy to maintain seeds in that range under a fluorescent lamp because it naturally gives off a gentle warmth in the right range. Trays placed under LED lights, cooler than fluorescent ones, though, may need the help of a heat mat. A heat mat can also be useful if you sprout seeds under natural light. Often a spot near an east or north window, where the light is right, is fairly cool, at least at night, and that can delay or even prevent germination.
Trays placed on heat mats. Photo: homesteadandchill.com.
Heat mats (readily available in garden centers and online) produce steady heat 24 hours a day and encourage fast and much more even germination. They’re inexpensive and most gardeners find they need only one. Just keep moving pots of seeds off the mat as they germinate and replace them with newly sown seeds.
You can germinate the rare seeds that sprout best under cool conditions, such as spinach and a few perennials, in a cool basement or heated garage.
After Seeds Sprout
When seeds sprout, it’s time to change their regime.
It’s best to remove the dome in stages so seedlings can get used to ambient conditions. Photo: getbusygardening.com
First, the relative humidity of almost 100% that prevails inside their mini-greenhouse dome is not only no longer necessary and can even be harmful. Seedlings (sprouted seeds) now need good ventilation, especially to help prevent damping off, a disease that can kill off entire trays of seedlings in just hours. When most of the seeds have germinated, remove the dome. To avoid any shock to the still fragile seedlings, do this in stages: first prop up one end to let in a little air, then the next day lift up the other end as well, then, on the third day, remove the dome entirely. Your seedlings will by then be acclimatized to the ambient humidity.
The gentle heat you provided to sprouting needs is no longer necessary after germination either. As odd as it may sound, although the majority of seeds germinate better under warm conditions, the seedlings they produce prefer things cooler, especially at night. So, remove the heat mat if you used one. Removing the dome will also reduce the temperature. If you place your trays of sprouted seeds near a window (more on doing that in the following paragraph), that spot will most likely be naturally cooler.
You’re looking for a daytime temperature of about 65 to 70 °F (18–21 °C), ideal for most seedlings, while the nighttime temperature could drop another few degrees, even as low 54 °F (12 °C). Leaf vegetables and root vegetables (cabbage, lettuce, onions, etc.) prefer even cooler conditions: no more than 65 °F (18 °C during the day) if possible and as low as 40 °F (5 °C) at night. Cool temperatures help prevent etiolation (seedlings stretching for the light).
Once the dome is removed, move seed trays to the sunniest spot you can find. Photo: thekiwicountrygirl.com
Finally, now that the seedlings are no longer covered with a dome that can trap heat and cause it to build up excessively (the well-known greenhouse effect), there is no longer any risk of overheating. Expose the seedlings to as much light as you can during the day by placing the trays in full sun or, at least, the most brightly lit spot you have available. Adjust the height of the fluorescent lights so that the top of the plants is approximately 4 to 6 inches (10 to 15 cm) from the tubes (6 to 12 inches/15 to 30 cm for LED lamps) … and then adjust the height of the lamp regularly as the seeds grow.
Photo: www.gardenbetty.com Question: I have a question regarding the germination phase of sowing seeds. Is light absolutely necessary or will heat and humidity be enough to get seeds to germinate? Then, when the stems begin to emerge from the soil, I assume it is time to supply light until I can acclimatize the young plants and…