The stems shot up, the tray fell over on itself, and what should be a dense carpet looks like tangled grass. The first thought is usually light, and it is usually right. The complication is that stretching under the blackout cover is a planned stage of the method, while stretching after the cover comes off is a defect, and the two look identical. What separates them is when the stem elongated and what made it long.
Quick glossary: Etiolation - growth in darkness or insufficient light: the stem elongates, no chlorophyll forms, the seedling is pale and thin. Hypocotyl - the stretch of stem between root and cotyledons; this is what gives microgreens their height and this is what stretches. Blackout - the stage of complete darkness after sowing, with the tray covered by a lid or an inverted tray.
Stretching Under Blackout Is the Goal, Not the Problem
In darkness a seedling searches for light and elongates its hypocotyl. The method uses this deliberately: darkness plus pressure from a lid or an inverted tray makes every seedling stretch at the same time and in the same direction. The result is an even carpet of uniform height with a firm hypocotyl that holds the plant upright after cutting.
Remove that stage and you get short seedlings of uneven height lying on the substrate and cutting badly. So the first thing worth checking when someone reports stretching is whether the problem appeared after the tray went under light, or whether the blackout simply has not finished yet.
How Long Blackout Actually Runs
The common figure of two to four days covers about half the catalog: blackout runs from 2-3 days (quinoa) to 6-10 days (parsley, celery, mint), while purslane, lettuce and tarragon need light from the start because their seed is photoblastic.
| Blackout length | Crops | Examples |
|---|---|---|
| 2-3 days | 1 | quinoa |
| 3-5 days | 35 | radish, broccoli, mustard, barley, cress, amaranth |
| 4-6 days | 14 | spinach, alfalfa, dill, beet, borage |
| 5-7 days or longer under a dome | 12 | cilantro, thyme, shiso, oregano, lemon balm |
| 6-10 days | 5 | parsley, celery, mint, sorrel, corn |
| special regime | 3 | purslane, lettuce, tarragon |
That last row explains a whole class of failures. Purslane, lettuce and tarragon have photoblastic seed: it germinates in light and stalls in full darkness. A standard four-day blackout gives these crops uneven germination or none at all rather than a stretched seedling. Their crop pages state the regime separately, and this is the case where the general rule does harm.
Corn sits in the table for a different reason: it is held in darkness for the entire cycle on purpose. The stretched pale yellow shoot is the product.
What Stretches a Seedling After the Cover Comes Off
Not enough light. The most common cause and the easiest to check. Microgreens work at 150-300 µmol/m²/s PPFD. The eye is no help here: a windowsill in winter looks bright and delivers a fraction of that. The numbers and the arithmetic are in the article on PPFD and DLI.
The lamp is too far away. PPFD falls with the square of distance. Raise a lamp to twice the height and the seedling gets a quarter of the light for the same electricity bill.
Uneven light across the shelf. The classic picture: the middle of the tray stands up, the edges lie down. This is checked by measuring uniformity at several points rather than taking one reading under the lamp.
Heat. Hypocotyl elongation speeds up with temperature. The same tray at 77°F (25°C) stretches more than at 68°F (20°C) even when nothing about the light has changed.
Dense sowing. A seedling shaded by its neighbors reads a shade signal and stretches upward even when the lamp above the tray is fine. This is a response to competition, and adding light treats it badly. It is treated by seeding rate.
Telling the Causes Apart
Causes are told apart by what the tray shows: even stretching across the whole tray points to too little light, stretched patches point to local over-seeding, and a long seedling with normal green color points to heat with light already sufficient. Check light first, then temperature, then density, in that order.
| What you see | Most likely cause | How to check |
|---|---|---|
| The whole tray evenly stretched and pale | too little light across the zone | PPFD at seedling level |
| Middle stands, edges lie down | uneven light distribution | readings at five points on the tray |
| Patches stretched, the rest normal | local over-seeding | a 1×1 cm window in two or three spots |
| Long seedling but normal green color | heat, light is sufficient | thermometer at tray level |
| Stretched and pale on day four under the lid | normal, blackout is still running | the crop page |
The order of checks matters. Measure light first, then temperature, then look at density. Reversing that order leads to cutting the seeding rate when the real cause is a weak lamp, and the next batch comes out sparse and just as stretched.
Three Mistakes That Cost the Most
Extending blackout so the seedling "gains height". The height arrives, and with it come pale color, a weak stem and mold risk in the damp dark. Height comes from the crop and the seeding rate, not from an extra day under the lid.
Raising the seeding rate to make the carpet look denser. A stretched tray reads as sparse, and the hand reaches for more seed. Denser sowing sharpens the competition for light and makes the stretching worse, while raising the risk of mold in the bottom layer.
Judging light by eye. Human vision adapts to the ambient level and gives no absolute reading. A quantum meter or a calculation from the lamp's data sheet settles the question in a minute; "looks bright" never settles it.
How to Know the Seedlings Are Standing Correctly
After coming into light the carpet evens out in color within a day and then holds vertical until cutting. Height is the same across the whole tray, edges included. The stem is springy: run a palm across the carpet and it bends and comes back rather than staying flattened. Green is uniform, with no pale patches. Above all, the blackout length you use comes from your own crop's page rather than from a general rule about four days.
If you want to go deeper: Yellow Microgreens: Etiolation, Deficiency or Overwatering takes on the other side of the same problem, where the question is the color of the seedling rather than its height, and the Days to Harvest Chart by Crop shows how much of each crop's cycle runs under light.