Improving Seed Germination via Incubation

Homemade Pepper Incubator
Homemade incubator uses LED grow lights and an incandescent bulb on dimmer to control temperature

Soil temperature and moisture play a role in maximizing pepper seed germination rates, as they do best under humid conditions with a soil temperature between 80-85°F during daytime hours. When starting seeds indoors during the winter, you'll experience the best results if you're able to provide humidity and raise soil temperature above normal thermostat settings.

While most pepper seeds will germinate under normal household conditions, some varieties are more finicky, like those from Capsicum chinense species. So while controlling your soil environment may not be absolutely necessary, it does help, and if you're not getting the results hoped for, that'd be one of the main things to consider.

For simplicity, this page is dedicated to optimizing the soil environment for sprouting pepper seeds and doesn't address choice of soil, watering or fertilization which are also important. Please see our Step by Step Growing Guide for more information on those topics.

While increasing humidity is as easy as covering the seed starts with plastic wrap, providing a better seed sprouting temperature, can range from easy and cheap to slightly less than $200 dollars (with a quality LED grow light) and some effort. The more costly and effort intensive methods typically provide better precision, and additionally, can even provide a sense of learning and accomplishment. While potentially not necessary, the more involved options can be a rewarding parent/grandparent and child project.

No Cost, Least Effort

Some areas of the house are typically warmer than others in the winter months. Simply placing planted seeds on top of the refrigerator can add a couple of degrees depending on the model and accompanying enclosure. Using a couple of different thermometers, we've measured a 2-5°F increase on ours.

Rooms with furnaces, water heaters, and/or 2nd refrigerators/freezers can also be quite warm. We have a basement room that measures 15x6 feet with a slotted door for airflow. In it, is our furnace, water heater and freezer. During winter, that room gets between 72-88°F on its own. The colder it is outside, the warmer it is in that room, due to the furnace kicking-on more often.

Heating Mats

To raise the soil temperature, a heating mat for seedlings is potentially the easiest and least expensive. The "for seedlings" part is important as they are designed to be water-resistant (though, not waterproof). Most manufactures claim their mats can provide a 10-20°F increase over room temperature. Combined with a timer, the heating mat can be automatically controlled to increase the soil temperatures during daytime hours. We usually shoot for 12-14 hours a day.

Reading product reviews is advised. Some heating mats are said to not distribute heat evenly across the mat, concentrating heat in the center. Others may be prone to get too hot and burn-out seedlings.

In doing this ourselves, we've decided to add in a temperature controller to improve precision, safety and save energy. The one we chose was about $35 and included power control for both heating and cooling. This allowed us to plug the heating mats into the heating power receptacle and a fan into the cooling side. We put the temperature probe firmly into the soil and set the temperature to 83°F with a 2°F heating/cooling margin.

We've placed a foam insulating board and a towel underneath. The foam helps to direct the heat upward toward the soil, while the towel is there to soak up any spills or seepage during watering. The mats are water resistant, so a little spillage shouldn't be a big issue, but the instructions warn against having the mats in standing water. Having the towel helps to wick any spilled water away from the mats.

We cover the cups with plastic wrap to trap evaporating water and increase humidity. This also helps to trap heat within the cups, making it easier to achieve our target temperature. The heating mats we're using aren't too hot and add around 10°F when the cups aren't covered. When covered with plastic wrap, the heating mechanism is more efficient and we're able to maintain the desired 81-85°F.

This is the configuration we're using, but we're not offering this as a recommendation. Heating pads can pose a safety risk if misused. Always read the instructions that come with your equipment and avoid doing anything they don't recommend or that they advise against.

Pros... Cons...
  • Doesn't take up much space.
  • Not too much effort to set up. If temperature control is used, once operational, there's very little maintenance.
  • Being open air, plants can be easily seen.
  • Without a temperature controller, consistently maintaining a soil temperature between 80-85°F can be tricky.
  • Uses more electricity.
  • Potential safety hazard if incorrect or poor quality mats are used, or proper ones are used incorrectly.

Climate Controlled Incubator

An enclosed incubator warms both the air and soil, while providing better ability to raise the humidity beyond typical winter household levels. When we decided to purchase our first set of LED grow lights, many of the reviews showed the lights being used in conjunction with grow tents. So we decided on a design that'd mimic a grow tent with the ability to control its temperature.

We created our box out of whiteboard-like material to reflect and diffuse light and wrapped it in radiant barrier insulation to trap heat. We use a 40W incandescent light bulb as the heating element with an inline dimmer switch as an adjustment to achieve 81-85°F. The LED grow lights are mounted onto a foam board and wrapped in the radiant barrier, serving as the top cover for the unit. Being enclosed, we added a fan to provide air-circulation, helping to prevent mold and harden the plants. The electrical components are controlled by a timer to provide light, heat, circulation for 12-14 hours per day.

This setup has produced around 90% germination rate and has been sufficient to mature our seedlings until they're ready to be brought outdoors. We'll usually plant the seeds in mid-January and start bringing them outside for hardening around mid-late March.

Pros... Cons...
  • Insulated enclosure traps generated heat, giving it the potential to be more energy efficient.
  • Relatively easy to increase and maintain humidity above household levels.
  • Need more dedicated space.
  • Being enclosed, it takes more effort to view and monitor plants.
  • Takes a decent amount of time and effort to build.

Our Preferred Method

The incubator was our first approach, a fun experiment, and served us reasonably well for many years. Though, we've since decommissioned it in favor of the heating mats. The heating mats are easier to implement, take up less room and are easier to monitor.

After Germination

Once seeds have sprouted, you can remove any covering that was used to retain humidity. Continue to keep the seedlings indoors and provide them with a good light source and airflow. See our Caring for Seedlings Guide for more information.

Additional Resources