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
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!
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, 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.
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
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.
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.
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).
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.
I hope this answers your questions.