Propagation & Seed Starting

How to Use Waterbed Heaters to Start Vegetable Seeds

If you want to start vegetables from seed, you might want to try using waterbed heaters as seedling heat mats. They provide warmth to the soil in the seed trays. This makes the seeds pop out of the ground just like it was June, even in the middle of February.

Waterbed heaters can be purchased online for very little money. This is much less costly than using a regular seedling heater. A regular seedling heat mat is more expensive, and it is not as versatile for keeping vegetable seedlings warm because of its limited size and low heat output.

When using water bed heaters for starting vegetables from seed, it is extremely important that you make sure the pad’s temperature bulb is underneath the seed tray before turning on the heating pad. The bulb provides feedback to the heat controller telling it that the tray above it has reached a certain temperature. If the bulb is not under the tray that is sitting on the heating mat, the controller will continue to put out heat until the mat burns out. If that happens, the water bed heater is ruined, and you will probably melt your plastic tray. Another caution is to start out on the lowest setting and see if that meets your needs for heat. The settings on the water bed heater dial are not as accurate as they might appear.

A simple and inexpensive digital indoor/outdoor thermometer can tell you whether the temperature inside the seed tray dome is acceptable. Use the external probe and set the thermometer to “outside”, and that will give you an idea of what your vegetable seedlings are seeing in the way of heat. Each seed packet will tell you the best temperature for germination. We set ours around 85 degrees, and that seems to work well for most of the seeds. Soil temperature is very important for starting vegetables from seed, so you might want to pick up a soil thermometer. Since the probe of a soil thermometer is difficult to insert and hold still in a peat pellet and the small cells of sheet pots, we just measure the soil surface temperature with the outdoor probe thermometer and estimate that the temperature of the soil an inch or so below that point is a few degrees warmer. The outdoor temperature probe on the thermometer is completely sealed, so you could place it under the soil for accurate readings, however inconvenient that might be. Regardless, once you find the temperature setting that works well with your waterbed heater controller, mark it so you don’t set it too high or too low.

The idea behind the waterbed heaters is two-fold. First, they are large enough to hold two standard 10-inch by 20-inch seedling trays, with just a little bit overhanging the edges of the heater. Second, they put out more heat than a standard seedling heater, and that means that they can be placed inside a “seedling house” or cold frame in the winter for starting vegetables from seed.

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