Why Are My Microgreens Growing So Slowly?

Microgreens are supposed to be fast. Radish in 7 days, broccoli in 10 days, sunflower in 12 days — these timelines are a large part of what makes microgreens appealing as a home crop. When your tray has been sitting under the light for two weeks and the shoots still look like straggly seedlings, something is slowing the plants down. The causes are usually easy to identify and fix, and understanding them will help you grow faster, more productive crops from your next tray onward.

Temperature Below the Optimum Range

Temperature is the most powerful driver of microgreens growth rate. The enzymatic processes inside the seed that trigger germination and the photosynthetic processes that drive post-germination growth are all temperature-sensitive. Most microgreens grow fastest at 20 to 24°C. At 16°C, the same variety may take 30 to 50% longer to reach harvest maturity. At 14°C, some varieties barely grow at all after germination.

A cold kitchen in winter is the most common cause of slow microgreens growth. The solution is to move the tray to a warmer spot in the house — a top shelf of a cupboard near a heat source, or close to a radiator (but not directly on it, as the heat and dry air can damage plants). Alternatively, a seedling heat mat raises the medium temperature and can transform growth rates in cold conditions.

Insufficient Light After Germination

Light deficiency slows growth dramatically after the blackout phase. Once seeds have germinated and the cover has been removed, the plant must switch from living off stored seed energy to producing its own energy through photosynthesis. Without adequate light, this transition is sluggish — the plant grows, but slowly, and the slow growth compounds over days into a noticeably delayed harvest. Increase light intensity or duration (aim for 14 to 16 hours per day) and raise the light closer to the canopy.

Inherently Slow Varieties

Some microgreens are genuinely slow and no adjustments will make them significantly faster. Basil takes 12 to 16 days under ideal conditions. Coriander takes 14 to 18 days. Celery and carrot microgreens take several weeks and are not recommended for impatient growers. If you want speed, stick to radish (6 to 8 days), broccoli (7 to 10 days) and peas (10 to 14 days). Know your variety's typical timeline before assuming something is wrong.

Old or Low-Viability Seeds

Seeds with reduced viability do not germinate and grow with the same vigour as fresh seed. Even if a tray germinates acceptably, plants from older seed may grow noticeably more slowly, produce smaller cotyledons and be more susceptible to stress. If you are consistently seeing slow growth across different varieties from the same supplier or seed packet, consider trying a fresh source of seed before changing anything else in your setup.

What to Do Right Now

Check the temperature of your growing space with a thermometer — if it is below 18°C, warm it up or use a heat mat. Check your light: bring the fixture within 8 cm of the canopy and extend the photoperiod to 16 hours. If both temperature and light are within range, check the variety's expected timeline against a reliable reference. In most cases, slow growth resolves when one of these three factors is corrected.