Gadenin日本語Get the appDarkness first, and it has to be real darkness
This crop is grown in two phases with opposite light requirements, which is what confuses people coming from ordinary seed sowing. For the first three to four days the tray is fully covered. A seedling in the dark puts everything into stem extension, and that stretch is what turns a squat seedling into a shoot with a long clean white stem you can cut and use.
Real darkness means no light leak. An upturned tray with drainage holes, a clear lid with a tea towel over it, or a box with a gap at one corner all let in enough light to stop the stretch on that side, and you end up with a mat that is an inch (2.5 cm) taller at one end. Use a solid cover, and if in doubt put the whole thing in a cupboard.
The stems that come out of the blackout are pale yellow-white and the seed leaves are folded and almost colorless. That is correct. Nothing has gone wrong and nothing needs rescuing.
Weighting the cover for the first forty-eight hours, with a book or a second tray holding it down, makes the shoots push against resistance and gives a noticeably more even mat. Take the weight off once the mat starts lifting it, or you will bruise the stems.
Uncover on height, not on the calendar
Uncover when the stems are one and a half to two inches (4 to 5 cm) tall, which in a room at 65 to 70F (18 to 21C) is day three or four. Take the cover off in a cool room on day three and the shoots are too short to have a useful stem; leave it on until day six and they are long, thin and permanently floppy.
The mat lifting the cover is the practical signal. When you can see the cover sitting visibly higher than it did the day before, look underneath, measure a stem against your finger and make the call.
Expect the tray to look worse for about two hours after uncovering. The shoots come out of high humidity into open air and wilt slightly. Bottom water immediately and they stand back up. This is the single moment in the run when a tray can be lost to simple neglect.
- Check the height under the cover Lift, look, and measure against a knuckle. One and a half to two inches (4 to 5 cm) of stem is the target, with folded pale seed leaves at the top.
- Bottom water before the light goes on Sit the tray in a quarter inch (6 mm) of water for ten minutes and pour off the excess. A dry tray meeting light for the first time wilts hard and some of it does not recover.
- Move it into indirect light, not full sun A bright spot out of direct beams for the first day. The mat has never seen light and a south-facing windowsill in the afternoon will scorch the tips.
- Give it full light from day two onward A bright windowsill, or a lamp 8 to 12 in (20 to 30 cm) above the mat for twelve to sixteen hours a day. Color comes up within a few hours and is fully green in two days.
How much light, and for how long
Greening up needs far less light than growing a plant does. The shoot is not being asked to build biomass from photosynthesis, only to make chlorophyll in the seed leaves. An ordinary bright room is sufficient, and any cheap LED shop light or desk lamp held 8 to 12 inches (20 to 30 cm) above the tray does the job in two days.
Twelve to sixteen hours a day is plenty. Running lights twenty-four hours does not speed up greening and does warm the tray, which is not a direction you want to move in with a dense wet mat.
Direct midday sun through glass is the one form of light to avoid. It heats the tray, dries the surface unevenly and burns the exposed seed leaves at the high points of the mat. A north-facing sill, a sheer curtain or two feet (60 cm) back from the glass are all better than the brightest spot in the house.
| Light source | Distance | Hours | Days to full green | Watch for |
|---|---|---|---|---|
| LED shop light | 8 to 12 in (20 to 30 cm) | 12 to 16 | 1 to 2 | Nothing, this is the easy option |
| Desk lamp with LED bulb | 10 in (25 cm) | 14 | 2 | Uneven coverage at the tray edges |
| Bright indirect windowsill | Set back from glass | Daylight | 2 to 3 | Leaning toward the window |
| Direct sun through glass | At the glass | Daylight | 1 | Scorched tips and a hot dry tray |
Leaning, and how to keep the mat level
Shoots greening on a windowsill lean toward the light within a day, and once a dense mat has leaned it stays leaned. It is not a disaster, it just makes cutting awkward, because scissors run level and a tilted mat gives you long shoots on one side and stumps on the other.
Turn the tray a half turn every twelve hours and the lean cancels out. Under an overhead lamp the problem does not arise at all, which is the main practical reason to use one.
If a tray has already gone over, cut it slightly higher than usual and take the shorter side in a second pass rather than trying to level it in one cut. You will lose a little length and nothing else.
Note the light setup with each tray
Light is the variable people change most often between trays and remember least accurately. Record where the tray sat, what the light was and how many days it took to go fully green, alongside the sowing date.
It answers a specific question that comes up every winter: whether your usual windowsill still works when the days are short. In most homes the answer is that greening takes a day longer from midwinter to early spring, and the log tells you that before a tray sits pale for three days and you start looking for a disease that is not there.