Step 3 - Seed Starting
The Best Seedling Heat Mats
The difference between peppers in ten days and peppers in four weeks. And the wrong tool for half the seeds you will sow.
Janet H. - the hands-on owner behind Janet Media Group.
Published - last updated
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Use a heat mat for peppers, eggplants, tomatoes and other warm-season crops, and take the tray off the moment they germinate. Do not use one for lettuce, brassicas or peas -- they germinate happily at room temperature, and warm soil actively inhibits lettuce.
Quick picks
| # | Product | Best for | Score | Price |
|---|---|---|---|---|
| 1 |  Top pickVIVOSUN Seedling Heat MatWaterproof mat sized for 1020 trays | Peppers, tomatoes and slow germinators | 8.8 | Check price on Amazon |
| 2 |  Bootstrap Farmer 1020 Seed TraysHeavy-gauge 1020 trays, reusable for years | Trays that last more than one season | 8.5 | Check price on Amazon |
| 3 |  Burpee Self-Watering Seed Starting KitCell tray, dome and watering mat in one box | First-time seed starting | 8.4 | Check price on Amazon |
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Scores are our own editorial judgment, formed from published specifications and independent sources. They are not aggregated customer ratings, and they are not the result of hands-on testing, which we do not currently do. See how we choose.
Soil temperature, not air temperature
Seeds respond to the temperature of the medium they sit in. A 68F room gives you soil somewhere in the low 60s once evaporation is taken into account, which is why a tray of peppers can sit doing nothing for three weeks in a perfectly warm house.
| Crop group | Wants | Heat mat? |
|---|---|---|
| Peppers, eggplants | Warm soil, around 80F | Yes -- the biggest difference of any crop |
| Tomatoes | Warm soil, around 75 to 80F | Yes, helpful |
| Cucumbers, squash, melons | Warm soil | Yes, though they germinate fast anyway |
| Brassicas, chard, beets | Moderate | No need |
| Lettuce | Cool -- germination is inhibited by warmth | No, and it is counterproductive |
| Peas, spinach | Cool | No |
Get a thermostat if you can
Without a thermostat a mat simply runs at its own output, which may be warmer or cooler than the crop wants and cannot be tuned. A thermostat with a probe pushed into the medium lets you set a target and hit it, which turns a rough tool into a precise one.
It also protects against the opposite failure. A mat in a warm room can push soil temperature past what the seed wants, and heat above the optimum slows germination rather than speeding it.
Take the tray off after germination
This is the instruction most people miss. Bottom heat after the seedlings are up encourages soft, stretched growth -- the same leggy result as poor light, and it compounds with it. It also dries the medium noticeably faster than you will remember to water.
Move the tray off the mat as soon as most seeds have emerged, and put the mat under the next sowing instead.
Watering changes on a mat
Warm soil evaporates faster. A tray on a heat mat can dry out in a day, and a dried-out tray of germinating seed is a lost tray -- once the radicle has emerged, drying kills it.
Bottom watering from a reservoir solves this better than anything, which is one reason self-watering kits and heat mats pair well. See the best seed starting kits.
The picks in full
Top pick
VIVOSUN Seedling Heat Mat
VIVOSUN
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Germination is driven by soil temperature, not air temperature -- and a heat mat is the difference between peppers in ten days and peppers in four weeks.
Seeds respond to the temperature of the medium they are in, not the room. Peppers and eggplants want soil in the 80s Fahrenheit to germinate briskly, and a 68-degree house gives them soil in the low 60s -- which is why they sit there for weeks.
A mat under the tray fixes that. Pair it with a thermostat probe if you can: without one the mat simply runs at its own output, which can be too warm for some seeds and is impossible to tune.
Take the tray off the mat once germination is done. Bottom heat after that encourages soft, stretched growth, and it dries the tray faster than you will remember to water it.
What works
- Dramatically faster germination for warm-season crops
- Waterproof, so spills are not a hazard
- Sized to standard trays
- Cheap to run
What does not
- Without a thermostat it just runs warm, with no target
- Dries trays out faster -- watering needs watching
- Should come off once seedlings are up
Skip it if: you only start cool-season crops. Lettuce, brassicas and peas germinate fine at room temperature and can actually be inhibited by heat.
| Type | Seedling heat mat |
|---|---|
| Sizing | Made to fit one or two standard 1020 trays |
| Construction | Waterproof |
| Thermostat | Sold separately on most models |
Trays that last more than one season
Bootstrap Farmer 1020 Seed Trays
Bootstrap Farmer
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Thick enough to pick up full without folding, which is the entire complaint about cheap trays and the reason these cost more.
Anyone who has carried a cheap 1020 tray full of wet compost knows the failure mode: it folds in the middle and the seedlings go on the floor. Heavy-gauge trays do not do that.
The 1020 name is a standard footprint, roughly 10 by 20 inches, and it is worth sticking to. Every humidity dome, heat mat, cell insert and shelf is built around it.
Buy a couple of solid, undrained trays alongside the drained ones. A solid tray underneath lets you water from below, which is gentler on small seedlings than watering from above and keeps the leaves dry.
What works
- Can be lifted full of wet medium without collapsing
- Lasts many seasons rather than one
- Standard 1020 footprint fits every dome and mat
- Solid trays double as reservoirs for bottom watering
What does not
- Several times the price of disposable trays
- Only worth it if you actually reuse them
- Heavier to store in quantity
Skip it if: you start one tray a year. The economics only work once you are reusing them season after season.
| Type | 1020 standard seedling trays |
|---|---|
| Gauge | Heavy-duty, substantially thicker than standard |
| Drainage | Drain-hole and solid versions sold |
| Compatibility | Fits standard domes, mats and inserts |
First-time seed starting
Burpee Self-Watering Seed Starting Kit
Burpee
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Everything in one box with a capillary mat underneath, which removes the single most common beginner failure: uneven watering.
Seeds fail for two reasons in a beginner's first tray: they dried out, or they were drowned. A capillary mat drawing water from a reservoir underneath removes both, because the medium takes what it needs and no more.
The dome is the other useful piece. It holds humidity during germination, which matters most for small seeds sown shallowly.
Take the dome off as soon as the first seedlings are up. Left on, it holds the still, damp air that damping-off fungus needs, and a tray can collapse overnight.
What works
- Capillary watering keeps moisture even without daily attention
- Humidity dome improves germination rates
- Nothing else to buy to get started
- Reusable for several seasons
What does not
- Cells are small, so seedlings need potting on quickly
- Kit price is high relative to buying components
- Dome must come off promptly once seeds germinate
Skip it if: you already have trays. Refill pellets and a dome cost a fraction of a new kit.
| Type | Self-watering seed starting kit |
|---|---|
| Cells | Sold in several counts, commonly 36 and 72 |
| Watering | Capillary mat and reservoir |
| Included | Tray, cells, dome, growing medium |
Common questions
Do I need a heat mat to start seeds?
Only for warm-season crops. Peppers, eggplants, tomatoes, cucumbers and squash germinate far faster with warm soil. Lettuce, brassicas, peas and spinach do not need it, and lettuce germination is actually inhibited by warm soil.
How long should seeds stay on a heat mat?
Until they germinate, then off. Bottom heat after emergence encourages soft, stretched growth and dries the tray fast. Move the tray off as soon as most seeds are up, and start the next sowing on the mat instead.
What temperature should a seedling heat mat be?
Around 75 to 80F at the medium for most warm-season crops, and up toward 80 to 85F for peppers. Without a thermostat you cannot target that -- the mat simply runs at its own output -- which is why a thermostat with a probe is worth the extra.
Can a heat mat be too hot for seeds?
Yes. Above a crop's optimum, germination slows rather than accelerating, and in a warm room an unregulated mat can overshoot. It also dries the medium quickly, and a germinating seed that dries out after the root has emerged does not recover.
Sources
Factual claims on this page are backed by the published sources below, and product specifications are the manufacturers’ own. Where a figure is not published, we say so rather than estimating it.