Going into 2020 we decided that a heated seed bed inside the polytunnel would be amazing for starting seeds early in the year without having to sacrifice our indoor space!
There are many plants that do not require such early start but we will be focusing on those such as Chilli Peppers and Sweet peppers that need a longer growing season and will really benefit from the extra time in the soil.
We’ve been using the bottom heat method for seed sowing for some years now and find it to be very effective, so to have a dedicated bed for this will be perfect.
Building the Raised Heated Seed Bed
The seed bed was designed to take up 2 bench tops, creating a bed around 3x10ft and leaves space for storage underneath.
The basic frame is made with some mdf boards for the base and recycled teak boards for the sides.
We lined it with strong (leftover polytunnel) plastic to protect the wood from rotting and prevent sand from falling out of any tiny gaps.
Time to drill a hole through one side for the heat cable to pass through. Then we cut some 1. 5inch thick insulation boards to fit snug inside.
Next we filled the bed with gritty sand mixture 2 inches deep and watered it through before laying out a heat cable.
We were able to recycle this cable which we found buried in a corner of the old greenhouse, once used for a small ‘cuttings’ bed. It was warped and almost solid after being piled up underground for over 12 years, so we warmed it up and used some pots to hold it down for a little while until it became bendy again. The cable is layed as close as possible without touching, to give the best results.
Once the cable is laid its time to complete the bed by topping up with an inch gritty sand and watering. This allows the heat to distribute evenly throughout the bed and is very important. There shouldn’t be any cable visible as it should be buried nice and evenly.
Thermostat temperature control
Monitoring the temperature
We are using simple waterproof thermometers to record the minimum and maximum temperatures on the seed bed as well as other areas inside the polytunnel and one outside for comparison. This will show what temperature the bed has dipped to overnight as when propagating seedlings we need to keep our bed at least 10c to avoid any damage to our tender seedlings!
This thermometer also displays humidity which is great for sprouting seeds. We use basic plastic propagation lids over the top to help create a nice humid environment inside.
More than just a seed bed
The heated bed can be used for various projects such as overwintering established tender plants, as well as housing young seedlings in Autumn. It will also make an excellent cuttings bed as it provides excellent rooting conditions for many plants.
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