How to Grow Microgreens on a Grow Mat: Seed to Harvest
Growing microgreens at home has never been more accessible — but the medium you choose makes a significant difference in your results. If you have heard about hemp grow mats and want to give them a try, this guide walks you through the complete process, from selecting your seeds to taking your first harvest.
Unlike general microgreens guides, everything here is written specifically for growing on TerraFibre hemp grow mats. You will find that the process is cleaner, simpler, and more consistent than growing in soil — especially once you have done it once or twice.
What you will need
Before you get started, gather the following:
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TerraFibre hemp grow mat (5" x 5" for a small tray, 10" x 20" for a standard 1020 tray)
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A grow tray with drainage holes or a raised-pattern bottom (for air circulation)
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A second solid tray for bottom watering
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Microgreen seeds — radish, broccoli, sunflower, or pea shoots are excellent for beginners
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A spray bottle or watering can with a gentle rose head
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A second tray or cover for the blackout phase
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A clean, well-lit space — a sunny windowsill or a basic grow light works well
|
TIP |
For your first grow, we recommend radish or broccoli. Both germinate quickly (within 2–3 days), anchor well into hemp fibre, and are very forgiving for beginners. Sunflower and pea shoots are also excellent but benefit from soaking seeds overnight before planting. |
At a glance: the full process
Here is a complete overview of the seed-to-harvest stages before we walk through each one in detail:
|
Step |
Stage |
What to Do |
Timeframe |
|
1 |
Preparation |
Soak your grow mat and set it in the tray |
15 minutes |
|
2 |
Seeding |
Spread seeds evenly across the mat surface |
5–10 minutes |
|
3 |
Germination |
Mist and cover — keep warm and dark |
Days 1–3 |
|
4 |
Blackout |
Stack a weighted tray on top to encourage upward growth |
Days 2–4 |
|
5 |
Light exposure |
Uncover and move to light — water daily from below |
Days 4–10 |
|
6 |
Harvest |
Cut above the mat surface when first true leaves appear |
Days 7–14 |
Step 1: Prepare your grow mat
Take your TerraFibre hemp grow mat and place it in your grow tray. The mat should sit snugly against the bottom without bunching or overlapping the sides.
Before seeding, you need to pre-wet the mat thoroughly. Hemp fibre has excellent water retention, but it does need an initial soak to become fully hydrated.
How to pre-wet your mat:
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Pour water slowly and evenly over the entire surface of the mat until it is fully saturated — the mat will darken visibly as it absorbs water.
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Tilt the tray gently to drain off any excess standing water. Your mat should be damp throughout but not sitting in a pool.
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If you are bottom watering (recommended), pour water into the solid bottom tray and allow the mat to absorb moisture upward for 10–15 minutes, then drain any excess.
|
PRO TIP |
Hemp fibre holds up to 1,050% of its weight in water, so it retains moisture well between waterings. Avoid over-saturating — you want the mat damp and even, not waterlogged. A good test: press lightly on the mat surface. If water pools on the surface, tip and drain. If it feels evenly moist like a wrung-out sponge, it is ready. |
Step 2: Seed your mat
Seeding density is one of the most important variables in microgreens growing. Too sparse and you waste tray space. Too dense and you risk poor airflow, which contributes to mold.
For a standard 10" x 20" tray, general seeding guidelines on TerraFibre mats are:
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Radish: 30–40g
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Broccoli: 25–35g
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Sunflower (shelled): 150–200g (soak overnight before seeding)
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Pea shoots: 200–250g (soak overnight before seeding)
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Wheatgrass: 200–250g
Seeding technique:
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Spread your seeds across the mat surface as evenly as possible. Work in sections — a small cup or your hand works well for distributing evenly or even a barbeque brush.
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Seeds should sit on the mat surface in a single layer, with minimal overlap. Some contact between seeds is fine; large clumps are not.
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Gently press the seeds down into the mat surface using a flat hand or the back of a spare tray. This encourages seed-to-mat contact, which improves germination rates.
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Give the seeds a light mist with your spray bottle — just enough to dampen the surface without washing seeds around.
|
NOTE |
Unlike growing in soil, you do not need to cover seeds with a layer of medium on a hemp mat. The mat surface provides all the contact and moisture the seed needs to germinate. Simply pressing seeds into the mat is sufficient. |
Step 3: The germination and blackout phase
This is the phase most beginners are surprised by — and it is where hemp grow mats genuinely shine compared to soil.
The blackout setup:
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After seeding, place a second tray directly on top of your seeded tray, upside down, to create a dark cover.
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Add some weight on top — a few books, another tray of water, or similar. The gentle downward pressure encourages seeds to anchor into the mat fibres and pushes stems to grow upward in search of light.
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Place your covered tray somewhere warm — ideally 18–24°C. A kitchen counter away from drafts works well.
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Check moisture daily. Lift the cover and mist lightly if the mat surface appears to be drying out. You should not need to water heavily during this phase if the mat was properly pre-wetted.
Days 1–3: You will see seeds swell, crack open, and send out small white radicles (root tips). These will begin gripping the hemp fibres almost immediately. By day 2–3, most species will have visible shoot emergence — tiny pale stems pushing upward.
Days 3–4: Shoots will have reached 2–4cm in height and will be firmly rooted into the mat. This is when you transition to the light phase.
|
COMMON QUESTION |
Should I water during blackout? In most cases, a well-saturated mat at seeding will hold enough moisture for the full germination phase (2–4 days) without additionalwater. If you notice the mat pulling away from the tray edges or the surface feels dry to the touch, add a small amount of water to the bottom tray and allow it to wick up. Avoid overhead watering during blackout as it can disturb germinating seeds. |
Step 4: Light and the grow-out phase
Once your seedlings are 2–4cm tall with strong white stems, it is time to introduce light. This triggers chlorophyll production — your pale yellow sprouts will begin greening up within hours of light exposure.
Light requirements:
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Natural light: a south-facing windowsill works well in summer. Rotate your tray 180 degrees daily to compensate for directional light and keep growth even.
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Grow lights: position 15–30cm above the tray surface. 12–16 hours of light per day is ideal. Most basic LED grow lights designed for seedlings work well.
Watering during light phase:
During the grow-out phase, bottom watering is strongly recommended on hemp grow mats. This means pouring water into your solid bottom tray and allowing the mat to wick moisture upward, rather than watering from above.
Bottom watering keeps the canopy dry, which significantly reduces mold risk — a critical advantage on any growing medium. The open weave structure of TerraFibre mats is particularly well suited to bottom watering, as the hemp fibres draw moisture upward efficiently and evenly.
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Water once daily in most home environments, or twice daily in warmer/drier conditions.
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Add 1–2cm of water to the bottom tray and check after 30 minutes — if all the water has been absorbed, add a small amount more. If water remains, tip it out and water again the next day.
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Never let your mat sit in standing water for extended periods.
Step 5: Harvest
Microgreens grown on TerraFibre hemp mats are ready to harvest when they have reached their cotyledon stage — where the first seed leaves have fully opened. For most species this is at 7–14 days, depending on species and growing conditions.
Signs your microgreens are ready:
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Cotyledon leaves are fully open and showing vibrant colour
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Plants are standing upright and uniform in height
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For species with true leaves (pea shoots, sunflower), the first true leaf may just be emerging
How to harvest:
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Use clean, sharp scissors or a harvest knife. Dull blades crush stems and reduce shelf life.
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Cut just above the mat surface — aim for 1–2cm above where the stem meets the mat. You do not need to cut at the mat itself; leaving a small stub helps keep your cut clean.
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Harvest the whole tray in one pass, working in rows from one end to the other.
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Transfer your harvested microgreens to a clean container. Rinse gently if needed, spin or pat dry, and refrigerate.
Shelf life: Freshly harvested microgreens stored dry in a sealed container will typically keep for 5–10 days in the refrigerator. Growing on hemp mats produces a cleaner cut with no soil contamination, which contributes to a longer, fresher shelf life.
|
PRO TIP |
For the cleanest harvest, do it in the morning before watering. Your plants will be upright, making for a neater cut and fresher-tasting greens. Avoid harvesting immediately after watering when stems are heavier and more prone to bruising. |
After harvest: what to do with your mat
One of the most satisfying things about growing on TerraFibre hemp mats is what comes after harvest. Unlike soil, there is no messy cleanup of substrate to dispose of.
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Remove the spent mat from your tray — roots and all. The root mass will have woven into the mat fibres, which is completely normal.
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Add the entire mat, including the root mass, directly to your compost bin. Hemp fibre is fully biodegradable and compostable, and the root mass adds nitrogen-rich organic matter to your compost.
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Rinse your grow tray with clean water — no scrubbing required. It is ready for your next grow.
There is no reusing a spent hemp grow mat — each mat is designed for a single grow cycle. This is by design: a fresh mat for each grow ensures optimal water retention, clean germination conditions, and no risk of pathogen carryover between cycles.
Troubleshooting common issues
Mold appearing on the mat surface
Some surface fuzz on stems and the mat itself is normal and is actually root hair, not mold — it has a dry, wispy appearance and disappears once exposed to light and airflow. If you see true mold (grey-green, powdery, with an odour), reduce watering frequency, improve air circulation, and switch to bottom watering exclusively.
Uneven germination
Usually caused by uneven seed distribution or dry patches in the mat. Ensure your mat is evenly saturated before seeding, and press seeds firmly into the mat surface to maximise contact.
Leggy or pale seedlings
A sign of insufficient light or too-long blackout phase. Move to light as soon as seedlings reach 2–4cm, and ensure your grow light is positioned close enough to the tray.
Seeds washing around when watering
Switch to bottom watering from the start, or use a very gentle mist setting on your spray bottle. Avoid pouring water directly onto freshly seeded mats.
Growing microgreens on a TerraFibre hemp grow mat is a genuinely straightforward process — especially once you understand the logic behind each stage. Pre-wet thoroughly, seed evenly, keep it dark and warm for germination, introduce light when your seedlings are ready, bottom water during grow-out, and harvest clean.
The hemp fibre does a lot of the work for you: holding moisture evenly, anchoring roots firmly, and delivering a clean separation at harvest. Once you have done your first successful tray, the process becomes second nature.
If you have any questions about growing on TerraFibre mats, our team is available at info@biocompositesgroup.com or through the contact page at terrafibre.ca.
