Growing microgreens like a pro means getting the environment just right. Light, temperature, and humidity are three critical factors that determine how healthy and productive your microgreens will be. Small tweaks in these parameters can make the difference between a weak, leggy crop and a dense, vibrant one. In this guide, we'll explain optimal ranges for light, temperature, and humidity, and provide practical tips on controlling each factor. With a dialed-in environment, you can achieve peak growth and consistent harvests every time.
Finding the Right Light Levels
Light is the engine of plant growth - through photosynthesis, it allows microgreens to develop rich color, nutrition, and form. While microgreens don't need intense full sun like mature plants, they do require plenty of light to grow strong. In fact, one common mistake is not giving microgreens enough light, leading to tall, spindly seedlings (a condition called etiolation or "leggy" growth). To avoid this, ensure your microgreens get adequate light duration and intensity each day.
How many hours of light? Most microgreens grow best with 12-16 hours of light per day. This extended day length ensures they photosynthesize enough to develop fully, since they are harvested so young. In nature, spring and summer days are long - we mimic that with grow lights on a timer. Aim for somewhere in that 12-16 hour window. More isn't always better; some growers even report that giving a few hours of darkness (for example, an 18 hours on / 6 hours off cycle) improves color and taste by letting plants have a rest period. Continuous 24-hour lighting is possible (microgreens don't absolutely require a dark period), but many find it unnecessary and potentially stressful to the plants. A good starting point is 16 on / 8 off, which many growers use with excellent results. Use an automatic timer so the schedule is consistent daily.
Light intensity and distance: Microgreens are generally considered a "low to medium light" crop - they don't need extremely high light levels like tomatoes or cannabis, but they do need more than a windowsill may provide. A useful metric is PPFD (photosynthetic photon flux density); often a range of 200-400 μmol/m²/s is cited as ideal intensity at canopy level for microgreens. If you don't have a light meter, focus on distance and wattage. Keep LED grow lights about 15-30 cm above the top of your microgreens. If lights are too far, the plants will stretch toward them and become thin. If lights are extremely close or powerful, you risk light burn or overheating (though with modern LEDs, heat is minimal). A simple test: observe your microgreens' behavior. If they are bending markedly toward the light or growing very tall and thin, it's a sign of insufficient light - you either need to bring lights closer or get a stronger light source. If you see any bleaching (yellowing or whitening of the leaves) or wilting at the top, the light might be too intense or too close - raise it a bit. Most of the time, lack of light is the bigger issue than too much, especially indoors.
Type of light: For peak growth, full-spectrum grow lights are recommended. These provide a balance of wavelengths including blue (for healthy, compact growth) and red (for development). Cool-white or daylight LEDs (around 5000K-6500K) also work well, as they mimic natural daylight. Avoid using warm household bulbs or very yellow light; plants respond best to a spectrum that looks like bright noon sunlight. If using fluorescent lights (like T5 fixtures), get ones labeled for plant growth or at least daylight spectrum. Also, ensure coverage is even - the entire tray should receive light. If you notice edges of trays are weaker (paler or taller) than the center, you may need to add reflective surfaces or additional lights to cover those areas.
Natural light considerations: If you're growing in a greenhouse or near windows, take advantage of the sun but be cautious of extremes. Direct outdoor sun in midsummer can actually be too intense and hot for tender microgreens, causing them to wilt. But generally, microgreens can thrive in sunny windows or greenhouses as long as temperatures are controlled. They are often called a "low-light" crop in the sense that they can grow with lower light than many veggies (Penn State notes a DLI below 20 mol/m²/day is sufficient). Just remember that "low-light" doesn't mean darkness - it means about what a bright indoor setup or a partially shaded greenhouse would provide. If relying on sunlight, adjust your sowing schedule to seasonal light (you may get faster growth in summer than winter due to day length and light intensity). You might supplement with grow lights in winter to maintain consistent output.
In sum, give your microgreens long, bright days. A common recipe for success is LED lights on for 16 hours daily at a fixed height (adjust as plants grow). Monitor your crop: deep green, stout microgreens mean the light is just right; pale or leggy ones mean they need more. Getting light right will reward you with robust, nutrient-packed greens instead of weak ones.
(For an in-depth look at lighting options and setup tips, check out our Lighting Setups for Consistent Growth article.)
Temperature: Keeping It in the Goldilocks Zone
Microgreens prefer it not too hot, not too cold - but just right. Most varieties originate from cool-season crops (like brassicas, peas) and sprout best in moderate temperatures. The general ideal range for growing microgreens is around 18-24 °C. Let's break down temperature considerations:
Germination phase: When you've just sown seeds and perhaps covered them (during the blackout or pre-germination period), a slightly warmer temperature can speed up sprouting. Many seeds germinate fastest around 21 °C. Using a seedling heat mat can help if your room is cold - placing trays on a heat mat set to ~21 °C can encourage quicker, more uniform germination. However, ensure you don't overheat - temperatures above 26-27 °C can actually harm germination for some seeds or encourage mold in the wet, closed environment. Typically, 20-22 °C is a sweet spot to sprout microgreens rapidly.
Growing phase: Once the microgreens are under lights and growing, maintain ambient air temperature around 18-22 °C for most varieties. At these temperatures, microgreens grow steadily without undue stress. If temps drop too low (say 10-15 °C), growth will slow and you'll add days to your harvest time; you may also see issues like yellowing if it's too cold for nutrient uptake. On the other hand, if temps rise too high (above ~27 °C consistently), microgreens can become leggy, overly transpire (losing water faster than they can take up), or even become susceptible to diseases. Warm and humid together is a bad combo - many fungi love warm (25 °C+) and wet conditions. A study on microgreens notes that optimal growth was observed in roughly the 17-20 °C range at night and slightly higher in day. So if you can mimic a mild spring day, you're in the zone.
Consistency: Just as important as the actual temperature is keeping it stable and consistent. Fluctuations can stress the plants. If your grow area swings from 18 °C at night to 28 °C midday, you might see uneven growth or even some shock. Try to avoid large swings (>5 °C difference) if possible. In practical terms, this might mean using a small heater in a garage setup overnight during winter, or venting out heat build-up from sun or lights during the day.
Cooling and heating tips: For indoor grows, most often the issue is excess heat from lights or lack of ventilation. Simple clip-on fans or an exhaust fan can pull out warm air and bring in cooler air, preventing heat accumulation. If you grow in a tent or small room, a thermostat-controlled fan system is great - it kicks on when a threshold is crossed. Conversely, in a cold basement or greenhouse in winter, consider insulating the growing area and using heaters carefully. Heat mats under trays can suffice if the ambient air is, say, 15 °C but the soil needs warming. They gently warm the root zone. Always monitor with a thermometer - don't guess! A digital min-max thermometer is inexpensive and lets you track how low and high it gets over 24 hours, so you can adjust.
Varietal differences: Most common microgreens (radish, broccoli, sunflower, pea, etc.) do fine in the aforementioned range. A few exceptions: micro basil and other herb microgreens (like shiso) enjoy the warmer end (~22-24 °C) and are a bit slower to germinate in cool temps - so give them warmth. Pea shoots actually prefer it a bit cooler; they can get fungal issues if too warm and wet. Some growers germinate pea at 16 °C and then grow under lights around 18 °C for stout, crisp shoots. So, if you are growing a variety that seems to struggle, check if its preferences differ. But in mixed operations, targeting ~20 °C is a safe middle ground that almost everything tolerates well.
Maintaining optimal temperature rewards you with faster growth and less disease. You'll notice microgreens in the right temp range stand upright and look perky, whereas too cold = slow and dull, too hot = droopy or stretched. Temperature control is a big part of professionalizing your microgreen production - it takes you from reacting to problems ("why are these not germinating?") to preventing them by keeping conditions optimal.
Humidity and Airflow: Managing Moisture in the Air
Humidity is a tricky factor: microgreens like moisture, but too much humidity invites mold. The goal is to keep the air comfortably humid but not stagnant. An optimal relative humidity (RH) for growing microgreens is around 50-70% during the growing phase. Here's how to manage it:
During germination (covered phase): When you've just sown seeds and perhaps stacked trays or covered them, humidity will be near 100% under the cover - that's good for quick sprouting. You actually want very high humidity in the immediate microenvironment to soften seed husks and get almost all seeds to pop. This is why people use domes or blackout lids. However, this phase only lasts 2-4 days typically. As soon as seeds have mostly sprouted and are ready for light, you must remove covers and decrease humidity, or else you risk fungal growth.
Growing phase humidity: Aim for ~50-60% RH in your grow area once microgreens are under lights. This range provides enough moisture that seedlings don't dry out too fast (microgreens have tiny roots and can wilt quickly in very dry air), but it's not so high that mold goes crazy. If humidity creeps above 70%, especially at warmer temperatures, you are likely to see mold or "damping off" fungus appear on the soil or stems (white fuzzy patches). In a closed room with many trays (which transpire water), humidity can naturally rise. Combat this with proper airflow and ventilation. Fans are your friend: a gentle oscillating fan or even a small PC fan on constantly will keep air moving around plants. This discourages fungus by preventing overly damp, still air from lingering on foliage. Good airflow also helps equalize temperature and humidity throughout the space.
If you find your ambient RH is regularly too high (for example, in a basement in summer), consider a dehumidifier. These devices can actively pull moisture out of the air and keep your grow room in the target range. Use a hygrometer (humidity meter) to monitor levels - many digital thermometers have RH reading as well. As a rule, if you ever see condensation on surfaces or consistently foggy windows in your grow space, the humidity is too high and needs reduction.
On the flip side, too low humidity (below ~40%) can cause issues too: microgreens may dry out at the tips, and you'll need to water more often. If you live in a very dry climate or are growing in winter with heated indoor air (which is usually very dry), you might need to add humidity. Ways to increase humidity include: misting the microgreens lightly with water a few times a day, placing open trays of water around (the evaporating water adds moisture to air), or using a humidifier set to a gentle output. Be cautious with humidifiers - clean them regularly, and don't overdo it. You want to nudge RH into, say, the 50% zone, not make it rainforest-level. Often simply having the moist soil and plant transpiration in trays is enough to raise humidity in a small space.
After watering cycles: You might notice right after watering your trays, humidity spikes (water evaporates into the air). This is normal but again highlights why air circulation is vital. Run fans to dissipate that extra moisture. If you bottom-water by soaking trays in water, there will be less direct evaporation into the air compared to heavy top-watering (another reason many prefer bottom-watering microgreen trays - it keeps surface drier and air drier, focusing moisture at roots where needed).
Night vs day RH: Sometimes when lights are off and temperature drops at night, humidity can rise (cool air holds less moisture, so RH % goes up). If you see a big RH jump at night, you might need to keep some airflow or slight heating at night. A small difference is fine, but you wouldn't want, say, 50% day and 90% night every cycle. Consistency again is beneficial for preventing disease.
In short, treat humidity and airflow as a pair - you manage them together to create a fresh, breathable environment for your microgreens. Many growers find that once they add fans and keep RH in check, previously chronic mold issues simply vanish. You'll also get sturdier stems; a gentle breeze from a fan makes microgreens sway slightly, which strengthens them (much like trees in wind). Just avoid pointing a very strong fan directly that could dry or chill the plants; indirect air movement is ideal.
Monitoring and Adjusting Your Environment
To truly fine-tune light, temperature, and humidity, you need to measure and monitor them. Invest in a few basic tools for your grow space: a reliable thermometer, a hygrometer, and a light meter (or at least use your phone's light sensor as a rough lux meter). Check these readings daily. Professional growers often keep logs of environmental conditions. By tracking, you can correlate any growth problems with environmental fluctuations and correct them.
Some tips for adjusting on the fly:
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If you see leggy, pale microgreens: increase light intensity or duration (e.g., move lights closer, or go from 12 to 16 hours) and ensure temperature isn't too high causing stretch. Also check that they weren't grown too long in blackout - introduce light a bit earlier next time to prevent over-stretching.
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If you see yellowing not due to lack of light (i.e., even the lit leaves are pale): possibly temperature is too low for that variety or humidity is too high causing root issues. Improve warmth and airflow, and ensure they got light at the right time.
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If you find white fuzzy mold on the soil or stems: immediately improve air circulation, reduce humidity, and check that you aren't overwatering. You can often salvage the crop by drying the surface (direct a fan at soil level). Long term, keep RH in recommended range and possibly sow a bit less densely if it's a recurring issue, since overcrowding can trap moisture.
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If edges of trays consistently underperform (like the outer microgreens are smaller): that could be light not reaching edges well or uneven temperature (edges might be cooler). Try adding reflective material (even aluminum foil or Mylar on walls) to bounce light, and ensure no cold drafts hit the tray edges. Rotating trays under the light can also even out growth.
Remember that different microgreen species might have slightly different ideal conditions. If you notice your salad mix of brassicas is thriving but your micro cilantro is struggling, you may need to tailor conditions or give the cilantro a separate area with warmer temps and a bit more light, for example. Over time, you'll learn the nuances.
Finally, know that peak growth comes from the sum of all factors being in balance. Light, temperature, and humidity are interconnected: warmer air holds more humidity; light can raise temperature; etc. Achieving a balance where, say, you have ~20 °C, 60% RH, and 16 hours of good light will create a microgreen-friendly climate where they practically "explode" out of the medium with growth. In such an environment, many microgreens can go from sowing to harvest in just 7-10 days, with minimal issues.
In conclusion, optimizing your microgreens' environment is one of the best investments of effort you can make. It reduces problems and boosts growth speed and quality. Instead of guessing, you'll be providing precisely what these young plants crave. So adjust those lights, set those thermostats, fine-tune your humidity - your microgreens will reward you with a beautiful, abundant harvest.
Soft CTA: Healthy microgreens start with a healthy environment - and healthy seeds. Make the most of your dialed-in grow room by using premium microgreen seeds from Deliseeds, ensuring vigorous growth right from the beginning. Happy growing!