How to Grow Microgreens Hydroponically: A Simple UK Guide

Growing microgreens hydroponically means skipping soil altogether: you sow seeds onto a moisture-retentive pad or plug tray, keep them dark and damp for two to four days to germinate, then move them under a light for 12 to 16 hours a day until you cut your first tray seven to fourteen days after sowing. No grow tent, no reservoir and no pump are required. A tray, a humidity dome, a growing medium and a small light are enough to get a windowsill or a shelf in the kitchen producing fresh pea shoots, radish, mustard or broccoli microgreens on a rolling weekly cycle.

It's one of the simplest entry points into hydroponic growing, and a useful way to get fresh, nutrient-dense greens on the table year round, including through darker months when outdoor salad crops have stopped. Below is how to set a tray up, which growing medium to pick, and how to avoid the two mistakes that ruin most first attempts: overwatering and starving the seedlings of light once uncovered.

What is hydroponic microgreen growing, and how is it different from soil?

A microgreen is simply a seedling harvested young, usually seven to fourteen days after sowing, once the first true leaves have opened but before the plant has put on any real size. Growing them hydroponically just means the seed sits on an inert, soil-free pad (a coir or peat plug tray, a rockwool micro-cube tray, or even plain kitchen paper) rather than in compost. There is no nutrient solution to mix for most microgreen crops: the seed carries enough stored energy to get to harvest on water alone, which is part of why this method is so beginner-friendly.

The main practical difference from soil is mess and speed. A hydroponic tray drains cleanly, doesn't attract soil gnats, and dries out evenly, so watering is more predictable. It's also the same basic principle used in hydroponic seed starting for transplants, except here the seedlings are never potted on. They're grown to a specific young stage and then cut, tray and all.

What do you need to get started?

A first microgreens setup is short: a shallow tray with drainage holes, a solid tray underneath to catch water, a growing medium, a clear humidity dome for the germination stage, seeds, a spray bottle, and a small light source for once the dome comes off. Everything except the seeds themselves can be bought as a single kit.

What's the best growing medium for hydroponic microgreens?

Any medium that holds moisture evenly and lets excess water drain away will work. The main options, in order of what most home growers try first:

For most people starting out, a pre-filled plug tray is the easiest route because there's nothing to mix or measure, and the cells hold moisture consistently without turning soggy.

How do you sow and germinate microgreen seeds?

Soak larger seeds such as peas and sunflowers for eight to twelve hours before sowing; smaller seeds like broccoli, mustard and radish can go straight onto the damp medium. Sow thickly and evenly so the surface is well covered but seeds aren't stacked on top of each other, then mist with a spray bottle rather than pouring water over the top, which can wash seeds into clumps.

Cover the tray with a humidity dome or a second tray placed upside down on top, and keep it somewhere dark for two to four days. This blackout period is what gets seeds to germinate quickly and push up straight before they're exposed to light. Check the medium daily and mist again if the surface starts to dry out. Room temperature germination usually takes three to five days; if your growing space is cool, especially once autumn brings the heating on and off through the day, a small heat mat under the tray keeps conditions steadier and speeds things up noticeably.

How much light do microgreens need once they've germinated?

As soon as you see pale yellow shoots pushing up under the dome, usually two to four days in, remove the cover and move the tray into the light straight away. Leaving them covered even one extra day makes them stretch thin and leggy. From this point they need 12 to 16 hours of light a day until harvest; a bright windowsill can work in summer, but a small LED bar gives far more consistent results, especially through fewer daylight hours later in the year.

Microgreens don't need a high-intensity full-spectrum panel built for flowering plants. A compact, low-heat propagation LED positioned 20 to 30cm above the tray is plenty, since the goal is simply to green up the leaves and build a bit of stem strength, not to maximise yield the way you would for a mature crop.

When and how do you harvest microgreens?

Most trays are ready seven to fourteen days after sowing, once the first true leaves have opened above the seed-leaf pair. Faster crops like radish and mustard are often ready around the seven to nine day mark; peas and sunflower shoots usually take a little longer, closer to ten to fourteen days. Harvest with clean scissors or a sharp knife, cutting just above the growing medium in one pass, and lift the whole mat of stems out together rather than picking individual seedlings.

Rinse harvested microgreens in fresh drinking water before eating, and only wash the amount you plan to use soon, since Penn State Extension notes that washing shortens how long they'll keep. Stored dry in the fridge, unwashed microgreens generally last several days longer than a washed batch.

Frequently asked questions

Do microgreens need nutrients or feed to grow?

No, for most quick microgreen crops the seed itself carries enough stored energy to reach harvest on water alone. Nutrient solutions become useful if you let a tray grow on past the microgreen stage into a full seedling, but for a standard seven to fourteen day tray, water is all that's needed.

Can I grow microgreens without a grow light?

Yes, a bright south or west-facing windowsill can produce a decent tray, particularly in spring and summer. Results get noticeably leggier and paler through autumn and winter as daylight hours drop, which is when a small propagation LED makes the biggest difference.

Do microgreens actually need to be grown hydroponically, or is soil fine too?

Soil works perfectly well for microgreens too. Growing them hydroponically on a plug tray or mat is simply cleaner, drains more predictably, and avoids the soil gnats and compost mess that put a lot of people off trying a second tray.

Can you get more than one harvest from the same tray?

Most microgreens are cut once and the tray is composted, since the stored energy in the seed is spent by harvest. A small number of crops, like pea shoots, will sometimes push a light second flush of growth if left watered after the first cut, but it's rarely worth counting on.

Why did my seedlings turn out pale and leggy?

This is almost always the blackout dome being left on too long after germination, or too little light once it comes off. Uncover the tray as soon as you see shoots emerging, and aim for a full 12 to 16 hours of light a day from that point.

How long do harvested microgreens keep in the fridge?

Unwashed and stored dry in a container or bag, most microgreens keep for around five to seven days in the fridge. Washing them before storage shortens that window, so it's best to rinse only what you're about to eat.