Growing microgreens in jars is the smallest hydroponic garden there is. A wide-mouth jar, a net cup, a disc of coir and a pinch of seed give you a little patch of greens on a windowsill, ready to cut in one to two weeks, with no pump, no tray and nothing to set up beyond the jar itself. It is a lovely way to try microgreens before committing a shelf to them.
It helps to be clear about what a jar can and cannot do. The growing area is only as big as the top of the net cup, so one jar gives a garnish, not a salad. Microgreens are really a tray crop, and the standard 10×20 inch tray is what feeds a household. A jar is for a sprinkle over tonight’s dinner, for a narrow sill, or for a child’s first grow. This guide covers how to set one up, the numbers that matter, and how a microgreen jar differs from a sprouting jar. If hydroponics is new to you, the apartment hydroponics beginner’s guide explains the basics.
Microgreens in Jars at a Glance
Key takeaways
- Use a wide-mouth jar with a net cup and a thin disc of coir or a rockwool slab, so small seeds cannot fall through the mesh.
- Start with the bottom of the cup in the solution, then let the level fall as the roots grow down. No pump is needed.
- Feed very little: plain water at first, EC 0.5 to 1.0 while pale, 0.8 to 1.2 once green, and never above 1.2.
- Radish is ready in 7 to 10 days, broccoli in 8 to 12, and pea shoots in 10 to 14. Each jar gives one cut.
- Light-proof the jar, keep it at 65 to 75°F, and give it 14 hours of light, since a windowsill alone often leaves the greens leaning and pale.
The Numbers for a Microgreen Jar
These are the figures the crop table on this site uses for microgreens, and harvest times count from sowing.
| Setting | Microgreens | Why it matters |
|---|---|---|
| pH | 5.5 to 6.5 | Since the crop is short, anywhere in this band works |
| Feed strength (EC) | Plain water first, 0.5 to 1.0 pale, 0.8 to 1.2 green | The seed feeds itself early on, so strong feed only burns |
| Light | 14 hours on, 10 off, 10 to 12 inches above the cup | When light comes from one side, the greens lean toward it |
| Light intensity | About 180 to 320 PPFD (DLI 9 to 16) | Modest, so a small LED bar over a row of jars is plenty |
| Water and room | 65 to 75°F (18 to 24°C), humidity 40 to 60 percent | Above 75°F, mold and damping-off become far more likely |
| Germination | About 1 to 4 days | Keep the cup dark and damp until the seedlings push up |
| Harvest | Radish 7 to 10 days, broccoli 8 to 12, pea shoots 10 to 14 | One cut per jar, at about 1.5 to 3 inches tall |
A Microgreen Jar Is Not a Sprouting Jar
The two get mixed up because both involve seeds and jars. They are different crops. Sprouts are seeds germinated in a jar of water with no medium and no light, rinsed a couple of times a day, and eaten whole, seed and root included, after a few days. Microgreens grow on a medium, under light, until they have green leaves, and you cut them above the medium and leave the roots behind.
That difference matters for flavor and for safety. Sprouts are grown warm and wet in the dark, which suits bacteria as well as seeds, and health agencies advise caution with raw sprouts for that reason. Microgreens are cut above the medium in light and airflow, and so they are a different proposition. Clean seed, clean jars and clean hands still matter with either. If a sprouting jar has kept going sour on you, a microgreen jar with a net cup, a light and a little air is the step up.
What You Need
One jar needs very little, and most of it you may already have:
- A wide-mouth jar. The wider the mouth, the bigger the growing area, because the top of the net cup is your whole crop.
- A net cup that sits in the mouth, or a wide-mouth lid with a net cup fitted. The mason jar net cups guide compares the options.
- A thin disc of coco coir, a slab of rockwool or a folded paper towel cut to fit the cup, so small seeds cannot slip through the mesh.
- A sleeve, paint or tape to make the jar opaque, because light in the water grows algae.
- Seed sold for microgreens or sprouting, which is untreated.
- A small LED on a plug-in timer if your window is not bright, plus a pH meter if you plan to feed.
The Kratky jar setup guide covers jars and lids in more depth, and why algae grows in jars explains why the jar must be dark.
Setting Up a Microgreen Jar
A microgreen jar works on the Kratky idea: roots drink from a still pool of solution below, and a layer of air opens up as the level falls. The whole setup takes a few minutes.
- Fill the jar with plain water adjusted to pH 5.5 to 6.5, so the bottom of the net cup sits in it, about half an inch to an inch deep.
- Set the damp disc of coir or rockwool in the cup, and let it wick up the water.
- Scatter the seed evenly over the disc. Seeds should touch but not pile up, since crowding in a small cup invites mold.
- Mist once, then cover the cup to keep it dark. An upturned small pot or a folded cloth works.
- Check daily and mist if the seed looks dry. Most seeds sprout in 1 to 4 days.
- Uncover as soon as the seedlings push up, and give them light the same day.
From there, the roots grow down through the cup into the water and the level drops as the greens drink. Do not top the jar back up to the cup, because that drowns the roots breathing in the air gap. A quart jar holds far more than a crop this short will drink, so it rarely needs anything added before harvest.
Feeding a Microgreen Jar
The seed feeds the seedling at first, so the jar can run on plain water for the first days. Once the leaves open, you can switch to a weak feed: EC 0.5 to 1.0 while the seedlings are still pale, and 0.8 to 1.2 once they are green. Never go above 1.2, because a strong mix on a crop this young burns the leaf edges instead of growing bigger greens. Mix the feed in a separate jug, since a small jar is easy to overdose. The pH and nutrient calculator works out tiny doses, and the nutrients roundup for small systems covers products that dissolve cleanly in small volumes.
Which Seeds Suit a Jar
Fast, forgiving seeds suit a jar best, because a small cup has little margin for mold or slow growth. Harvest times from sowing:
- Radish, at 7 to 10 days: the quickest and the most forgiving, with a peppery bite.
- Broccoli, at 8 to 12 days: mild and fresh, a good everyday topper.
- Pea shoots, at 10 to 14 days: sweet and crunchy, although the big seeds need a soak and fill a small cup quickly.
- Micro herbs such as basil, at 16 to 28 days: possible, but slow, and their seed turns to gel when wet, which holds moisture against the stems.
Grow one kind per jar, since each is ready on its own day. For a full tray instead of a jar, see radish microgreens in a shallow tray and pea shoots on a windowsill. For a mix, set up several jars and combine the cut greens; the best plants for one bright window suggests other easy crops for the same sill.
Light on a Windowsill
A windowsill is the natural home for a jar, but the light comes from one side. Microgreens lean hard toward the glass, and on a gray day they stretch pale and fall over. Turn each jar a half turn every morning, and use your brightest window. South and west windows work best.
For straight, green growth, add a small LED above the jars: 14 hours on and 10 off, with the light 10 to 12 inches above the cups. Microgreens need only about 180 to 320 PPFD, so a basic bar covers a whole row of jars. Treat the height as a starting point for a typical home LED. Lower it an inch if the greens stretch, and raise it if the leaves look bleached. Window light plus a grow light shows how to combine the two, the clip-on grow light guide covers the simplest lamps, and the compact timers roundup keeps the 14 hours honest.
Temperature and Humidity
Keep the jars at 65 to 75°F (18 to 24°C). A sunny sill in summer can heat a small jar of water fast, and above 75°F mold and damping-off become far more likely, so move jars back from hot glass in a heatwave. In winter, a sill right against cold glass slows everything down. Humidity is best at 40 to 60 percent, because microgreens packed into a small cup hold damp air against themselves. A small fan nearby, or simply a spot with some air movement, helps. Small system temperature fixes covers both seasons.
Harvesting and Keeping Jars Going
Cut when the greens stand about 1.5 to 3 inches tall and the first leaves have opened, within the windows above. Use sharp scissors and cut just above the disc, when the leaves are dry. A quick swish in cold water floats off any seed hulls.
Each jar gives one cut. The stems do not grow back, so compost the disc and roots, pour out the water, wash the jar and cup with soapy water, and wipe them with diluted hydrogen peroxide before the next sowing. The cleaning guide covers the routine.
For a steady trickle of greens, run jars in rotation: sow one or two new jars every few days, so there is always one ready. The same idea scales up to herbs in succession planting, and the seed to harvest calculator gives you dates for each sowing.
Where these facts come from
🔗 Where these numbers come from
The pH, EC, light, temperature, humidity and harvest figures here are the ones the crop table on this site uses for microgreens and micro herbs, and the crop sources page lists the research behind them. Virginia Cooperative Extension’s guide to microgreens indoors covers light and production, and the Utah State Extension guide to growing microgreens covers home growing. Using a jar and net cup instead of a tray, and the light height in inches, are this site’s own practice.
Troubleshooting Microgreen Jars
Small cups show problems quickly, so it pays to recognize them early.
| What you see | Most likely cause | What to do |
|---|---|---|
| Seeds falling into the water | No disc in the cup, so small seeds slip through the mesh | Line the cup with coir, rockwool or a paper towel |
| Greens leaning hard toward the window | Light from one side only | Turn the jar daily, or add a light overhead |
| Tall, pale stems falling over | Too little light, or uncovered too late | Uncover as soon as they push up, then lower the light |
| Gray web spreading across the seeds | Mold, when sowing is too dense and the air too still | Sow thinner and add airflow, since humidity should sit at 40 to 60 percent |
| Green film in the water | Light getting into the jar | Sleeve or paint the jar before the next sowing |
| Brown leaf edges | Feed stronger than 1.2 | Use plain water or a weaker mix next time |
Mold or Root Hairs?
In the first days, fine white fuzz around the roots is usually root hairs, which vanish when misted. A gray or white web spreading over several seeds, often with a musty smell, is mold. In a jar it usually means the seed was sown too thickly or the disc is sitting in too much water. Lift the cup a little so the disc only wicks, sow thinner next time, and keep air moving. If seedlings pinch at the base and collapse, that is damping-off, which cannot be rescued: compost the jar’s contents and clean everything. The guide to avoiding damping-off goes deeper, and tiny flies around the jars are covered in the fungus gnat guide.
One Last Tip
Use your jars to learn, then move to a tray for volume. A jar shows you, up close and in a few days, what dark germination looks like, when root hairs appear, how fast the greens turn toward light and exactly when radish is at its best. Those are the same lessons a tray teaches, at a fraction of the seed cost. Once you have cut three or four good jars, you know enough to fill a tray and grow real quantities with confidence.
Frequently Asked Questions
Can you grow microgreens in a jar?
Yes. Set a net cup in a wide-mouth jar, line it with a thin disc of coir or rockwool, and sow the seed on top with the bottom of the cup in the water. The roots drink from below with no pump. A jar gives a garnish-sized harvest, while a tray grows real quantities.
What is the difference between sprouts and microgreens in a jar?
Sprouts germinate in water with no medium or light and are eaten whole, seed and root included. Microgreens grow on a medium under light until they have green leaves, and are cut above the medium. Sprouts are grown warm and wet in the dark, which is why health agencies advise care with them raw.
How long do microgreens in a jar take to grow?
Radish is ready in 7 to 10 days from sowing, broccoli in 8 to 12 and pea shoots in 10 to 14. Most seeds sprout within 1 to 4 days under a cover. Micro herbs such as basil take much longer, 16 to 28 days.
Do microgreens in jars need nutrients?
Only a little. Plain water works for the first days, because the seed feeds the seedling. Once the leaves open, use EC 0.5 to 1.0 while they are pale and 0.8 to 1.2 once green, never above 1.2. Mix the feed in a jug first, since a small jar is easy to overdose.
Should I top up the water in a microgreen jar?
Usually not. Start with the cup bottom in the solution and let the level fall as the roots grow down, because the roots breathe in the air gap that opens up. A quart jar holds more than a crop this short will drink. Never top it back up to the cup.
Will microgreen jars grow on a windowsill without a grow light?
On a bright south or west sill, yes, if you turn each jar every morning so the greens do not lean. On a dim sill or in winter they stretch pale and fall over. A small LED 10 to 12 inches above the jars for 14 hours a day gives straighter, greener growth.
What are the best microgreens to grow in a jar?
Radish is the best start, ready in 7 to 10 days and forgiving. Broccoli is a mild alternative, and pea shoots work if you soak the seed. Micro herbs like basil are possible but slow, and their gel-coated seed holds moisture against the stems.
How do I stop small seeds falling through the net cup?
Line the cup with a thin disc of coco coir, a slab of rockwool or a folded paper towel before sowing. The liner holds the seed, wicks water up from the jar, and gives the roots something to grow through on their way down.
Do microgreens in a jar grow back after cutting?
No. Each jar gives one cut, because the stems do not regrow. Compost the liner and roots, wash and disinfect the jar and cup, and sow again. Sowing a new jar every few days keeps a steady supply of greens on the sill.
Why is my microgreen jar growing mold?
Usually because the seed was sown too thickly, the liner is sitting in too much water, or the air is damp and still. Sow thinner, lift the cup so the liner only wicks, and keep humidity at 40 to 60 percent with some air movement. Fine white root hairs in the first days are normal.
