Best Grow Lights and Heat Mats for Late-Winter Seed Starting

What each piece of equipment does

A heat mat and a grow light solve two separate problems. The mat warms the potting mix so seeds sprout, and the light keeps the seedlings short and sturdy once they are up. For most home setups the best pairing is a mat with a thermostat and a broad-spectrum LED or fluorescent fixture that you can raise and lower. University of Maryland Extension keeps such a fixture 2 to 6 inches above the plants for 14 to 16 hours a day, and University of Maine Extension takes the tray off the mat once about 50 percent of the seeds have sprouted.

The two do not need to cover the same area. Seeds do not need light to sprout in most cases, and Penn State Extension notes that most seeds germinate best in the dark. A tray therefore sits on the mat for a short stretch and under the light for weeks. One mat can serve several trays in turn, while every tray you have growing needs its own lit space until it goes outside.

Which seeds need a heat mat

Whether a mat is worth buying depends on your seed list. University of Maryland Extension puts a potting mix temperature of 65 to 75°F as good for germinating most vegetable seeds, which a heated living room already provides. A cool basement or spare room is a different case, and so are the warm-season crops.

Penn State Extension publishes soil temperature ranges for germination. The optimum range for each crop is wide, but the single best temperature inside it sits high for the fruiting vegetables.

Crop	Minimum	Optimum range	Best temperature
Lettuce	35°F	40 to 80°F	75°F
Onion	35°F	50 to 95°F	75°F
Cabbage	40°F	45 to 95°F	85°F
Tomato	50°F	70 to 95°F	85°F
Pepper	60°F	65 to 95°F	85°F
Eggplant	60°F	75 to 90°F	85°F
Cucumber	60°F	60 to 95°F	95°F

Read the table against the temperature of the room where the trays will sit. Lettuce and onions sprout in mix as cool as 35°F and do not need bottom heat anywhere in a house. Eggplant has the narrowest window, 75 to 90°F, and tomatoes want at least 70°F, so those two and peppers are the trays to put on the mat first. Lettuce also has the lowest maximum Penn State lists for these seven crops, 85°F, which makes it a poor candidate for a mat set up for peppers.

How to choose a seedling heat mat

The main choice is whether the mat has a thermostat. University of Maine Extension describes inexpensive bottom heat mats as warming the growing medium to about 70°F. That is the low end of the tomato range and below the 75°F floor for eggplant. Penn State Extension points to germination mats that let you set the temperature according to what the seed requires, and that control is the feature to pay for if peppers and eggplant are on your list.

Mat setup	Suits	Limit
Basic mat, no controller	Tomatoes and most vegetables in a cool room	Warms mix to about 70°F and cannot be adjusted
Mat with a thermostat	Peppers, eggplant and other crops with an 85°F optimum	The probe has to sit in the mix to read what the seeds feel
Mat sized for several trays	Sowing several flats in the same week	Every tray on it gets the same temperature

Size matters less than it seems, because the mat is in use for such a short time. Maine’s rule is to remove the tray from bottom heat as soon as about 50 percent of the seeds have sprouted, take off the plastic cover and move the tray under lights. The same publication points out that this frees the mat to sprout several more trays within a few weeks. A single-tray mat can carry a whole season of sowings if you stagger them.

How to choose a grow light

Seedlings stretch when they do not get enough light, and a winter windowsill rarely supplies it. The University of New Hampshire Extension fact sheet on growing seedlings under lights works from a target of 12 moles of light per square meter per day. A fixture reaches that total through a mix of brightness, distance and running time, so the practical question for any light is how close it must hang and how long it must run.

Light type	Height and hours reported	Source
Fluorescent shop fixture, two 40-watt tubes	4 to 6 inches above seedlings, 12 to 14 hours	Iowa State
Fluorescent tubes on a timer	2 to 4 inches above the canopy, 16 hours	Maine
Four-bulb T8 fluorescent shop light	Less than one foot away, 22 hours for sun-loving plants	New Hampshire
LED light bar	8 hours at 8 inches, or 16 hours at 20 inches	New Hampshire
Broad-spectrum LED or cool-white fluorescent	2 to 6 inches above plants, 14 to 16 hours	Maryland
Red and blue (purple) horticultural LED	May need about 10 hours for growth that takes a shop light 22	New Hampshire

The sources do not agree on one number, and the table shows why. Each tested or described a different fixture. The New Hampshire figures are the most useful for comparing types, because they hold the light target constant. One LED light bar met it in 8 hours from 8 inches away. The same bar needed 16 hours from 20 inches. A four-bulb T8 shop light needed 22 hours from under a foot.

For color, Maryland recommends broad-spectrum LED grow lights that mimic daylight, or fluorescent tubes in cool white with a 6500K color temperature. Iowa State pairs one cool white tube with one warm white tube in each fixture. A red and blue LED can reach the light target in fewer hours, as the New Hampshire figures show, but white light makes it easier to see the true color of the leaves when you check for yellowing or pests. The guide to grow lights for vegetable seedlings compares the fixture types in more detail.

Three features matter more than the label on the box. The fixture should be at least as long as your seed starting trays, so the end cells are lit. It should hang from chains or adjustable cords, since every source above expects you to raise it as the plants grow. And it should plug into an outlet timer, which is how Maryland and Clemson both run their lights.

Running the mat and the light together

Start with the tray covered and on the mat, lights off or on as convenient. Check each day. When about 50 percent of the cells show sprouts, move the tray off the heat, remove the cover and switch to the light. University of Wisconsin Extension gives the reason for turning the heat off: once germinated, most seedlings grow better with fluctuating day and night temperatures.

After that, cooler air is an advantage. Maryland suggests a night temperature of 55 to 60°F and a day temperature of 65 to 70°F for seedlings, and Maine gives 55 to 70°F as the ideal ambient range.

Hold the light at the height your fixture type calls for. Clemson keeps a 2 to 4 inch gap between fluorescent tubes and the plants, starting just above the soil line and raising the light as the transplants grow.

Water and air finish the job. Maryland waits until the top of the mix is nearly dry before watering. Maine waters from below by setting the containers in a tray of 1/2 inch of water for 20 minutes. A small fan blowing a gentle breeze across the seedlings improves air circulation and toughens the stems, and Wisconsin notes that it helps protect against damping off. Clemson adds the fan once plants have two true leaves. Running a hand gently over the tops each day has a similar effect on the stems.

Fertilizer can wait. According to Maryland, most potting mixes contain enough fertilizer to grow a seedling for at least 6 weeks. In a soilless mix with no starter charge, Clemson begins feeding when the first true leaves appear. Maine uses a half-strength, water-soluble, all-purpose plant food when seedlings are moved into a new container.

Matching the setup to your seed list

A list of lettuce, onions and cabbage needs light and no mat in a normally heated house, because the optimum range for each of those starts at 50°F or lower. Put the whole budget into a fixture long enough for the trays.

Add tomatoes and a basic mat becomes useful in a cool room, since about 70°F is the bottom of their optimum range. Add peppers or eggplant and a thermostat set near 85°F is worth having, because that is the best germination temperature Penn State lists for both.

Whatever the list, plan lit space for the largest number of trays that will be growing at one time. The seed packet gives the number of weeks to sow before the last frost date for each crop, and counting back from your last frost date shows which trays overlap. Before they move outdoors, seedlings raised under lights also need a gradual introduction to sun and wind, and the schedule for hardening off seedlings covers that step.