You do not need trays, racks or special equipment to start growing greens indoors. A clean glass jar, a little potting mix and a pinch of seed is enough for a small, pretty harvest on your counter. This guide shows you how to grow microgreens in a jar: how deep to fill it, how much seed to use for a jar instead of a full tray, how to water without drainage holes, and which crops work best. It is a good first project if you want to try microgreens before investing in a full setup.
Key takeaways
- Use a wide-mouth jar (pint or quart) so you can reach in to harvest and air can move.
- Add about 1.5–2 inches (4–5 cm) of moist potting mix or coco coir; no gravel layer needed.
- A wide-mouth jar has roughly 1/20 of the growing area of a 10×20 tray, so use only about 1–2 g of seed.
- Water sparingly with a spray bottle or spoon; the jar has no drainage, so soggy soil is the main risk.
- Fast crops such as radish, broccoli, cress and mustard are ready in about 7–12 days.
Microgreens in a jar vs. sprouts in a jar
Search for “growing greens in a jar” and you will mostly find sprouting instructions. Those are a different method. Sprouts are seeds rinsed in a jar with water only, eaten whole (seed, root and shoot) after a few days. Microgreens grow in a medium such as soil, under light, and you cut only the stems and leaves above the soil. Our article on microgreens vs. sprouts explains the differences in more detail. This guide is about true microgreens grown in soil inside a jar.
What you need
- A wide-mouth glass jar. A pint (16 oz) or quart (32 oz) canning jar works well. Narrow jars are hard to harvest from and trap humid air.
- Growing mix. A fine, peat- or coir-based seed-starting mix, or plain coco coir. Avoid garden soil, which compacts and can bring in pests. See the best growing medium for microgreens.
- Seeds sold for microgreens or sprouting (untreated). About 1–2 g per jar.
- A spray bottle and a small spoon or measuring spoon for watering.
- A bright window or a small LED light.
- A small saucer or piece of cardboard to cover the jar during germination.
How much seed to use in a jar
The most common jar mistake is using far too much seed. A standard 10×20 tray has roughly 190–200 square inches (about 1,250 cm²) of growing area. The inside of a wide-mouth pint jar is only about 3–3.5 inches (8–9 cm) across, which works out to roughly 7–9 square inches (45–60 cm²). That is about one-twentieth of a tray. Divide your usual tray rate by about 20:
| Crop | Seed per 10×20 tray | Seed per wide-mouth pint jar | Harvest | Jar-friendly? |
|---|---|---|---|---|
| Radish | 25–35 g | 1.5–2 g (about 1/2 tsp) | 7–10 days | Excellent |
| Broccoli | 15–20 g | About 1 g (1/4 tsp) | 8–12 days | Excellent |
| Cress | 15–20 g | About 1 g (1/4 tsp) | 8–12 days | Excellent |
| Mustard | 15–25 g | About 1 g (1/4 tsp) | 8–12 days | Very good |
| Basil | 15–25 g | About 1 g | 14–21 days | Fair (slow; humid jar raises mold risk) |
| Sunflower, pea shoots | 150–300 g | Not recommended | 9–14 days | Poor: large seeds need more room and weight |
Teaspoon equivalents are rough, because seed size varies. A small kitchen scale that reads 0.1 g is the most accurate option. You can also use our microgreens seed calculator with your jar’s area, or check the full seed-per-tray chart for other crops.
How to grow microgreens in a jar step by step
Step 1: Clean the jar
Wash the jar in hot, soapy water, rinse well and let it dry. A clean jar reduces the chance of mold in such a small, enclosed space.
Step 2: Add moist growing mix
Moisten the mix in a bowl first until it feels like a wrung-out sponge. Spoon 1.5–2 inches (4–5 cm) into the jar and press it gently flat with the bottom of a small glass. You do not need a layer of gravel or pebbles at the bottom. It does not create real drainage; it only reduces root space. Careful watering is what keeps the jar healthy.
Step 3: Sow the seed
Sprinkle the seed evenly across the surface so seeds are close but not piled up. Mist until the seeds look wet. Most small seeds, like radish and broccoli, do not need soaking. If you want to learn when soaking does help, read our guide to soaking microgreen seeds.
Step 4: Cover for 2–3 days
Rest a saucer or a piece of cardboard on top of the jar to keep it dark and humid. Keep it at room temperature, ideally 65–75°F (18–24°C). Mist once a day if the surface looks dry. When most seeds have sprouted and the seedlings are pushing upward, usually after 2–3 days, remove the cover. This is a mini version of the microgreens blackout period.
Step 5: Give it light
Move the jar to your brightest window, or place it under a small LED light for 12–16 hours a day. Turn the jar a half turn each day on a windowsill so the seedlings do not lean toward the glass. Light coming through the side of the jar also helps the stems green up evenly.
Step 6: Water carefully (no drainage)
Because the jar has no holes, extra water has nowhere to go. Use these rules:
- Add 1–2 teaspoons (5–10 ml) of water at a time, poured gently along the inside edge of the glass so it soaks in from the side.
- Water only when the top of the mix looks lighter in color or the jar feels lighter in your hand. In most homes that is every 1–2 days.
- Look through the glass: if you see water pooling at the bottom, stop watering until it is gone.
- Avoid soaking the leaves once seedlings are up; wet foliage in a jar dries slowly.
Our microgreens watering guide explains how to read moisture levels in more detail.
Step 7: Harvest (day 7–12)
Harvest when the first pair of leaves is fully open and the greens are about 2–3 inches (5–8 cm) tall. Gather a small bunch with your fingers and cut just above the soil with small scissors. One pint jar typically gives a small handful, enough to top a sandwich, a bowl of soup or a couple of salads. For tips on washing and storing, see how to harvest microgreens. Then empty the jar into the compost, wash it and start again, since most microgreens do not regrow.
Troubleshooting microgreens in a jar
| Problem | Likely cause | Fix |
|---|---|---|
| Water pooling at the bottom | Overwatering with no drainage | Stop watering; tip the jar gently and drain excess; water in smaller amounts |
| Gray fuzz or sour smell | Too wet, too much seed, still humid air | Use less seed, water less and keep the jar uncovered after germination |
| Tall, floppy stems | Not enough light | Use the brightest window or a small LED for 12–16 hours |
| Seedlings leaning to one side | Window light from one direction | Turn the jar a half turn every day |
| Patchy germination | Seed dried out or sown too deep | Keep the cover on and mist daily until sprouted; sow on the surface |
| Tiny flies around the jar | Fungus gnats in damp soil | Let the surface dry slightly between waterings; see our fungus gnat guide |
White fuzz on the roots is not always mold. Healthy radish and broccoli roots grow fine white root hairs that look fuzzy; our guide to mold vs. root hairs shows the difference. For small flies, see fungus gnats in microgreens.
Frequently asked questions
Can you grow microgreens in a mason jar without soil?
Yes, on a disk of hemp mat or a few layers of paper towel cut to fit, but moisture is harder to manage without drainage. Soil or coir is more forgiving in a jar.
Do I need drainage holes to grow microgreens in a jar?
No. Microgreens grow for only 1–2 weeks, so careful, small waterings are enough. Drainage holes help most when people overwater.
What are the best microgreens to grow in a jar?
Small, fast seeds such as radish, broccoli, cress and mustard. Our list of the easiest microgreens to grow has more beginner-friendly options.
How many jars should I start?
For a steady supply, start one new jar every 3–4 days. That way a fresh jar is always ready while the next one grows.
A jar is a simple, low-cost way to learn how microgreens behave. Once you are comfortable, moving up to a full tray is easy; our step-by-step guide on how to grow microgreens shows you how.
