Polytunnel Progress: Building a Heated Seed Bed

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!

DIY propagation bed

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 strong mdf boards for the base and recycled teak boards for the sides.

We lined it with plastic to protect the wood from rotting and prevent any sand from falling out of 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 up to 4inches and watered it through before laying out a heat cable.

Heating up the 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 with a sprinkler system.

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 before covering.

Once the cable is laid its time to complete the bed by topping up with gritty sand and watering. This helps the heat to distribute evenly throughout the bed. You shouldn’t have any cable visible as it should be buried nice and evenly.

Thermostat temperature control

Warming up the bed

We will be using the heated element during the coldest months of the year so it will be set on maximum temperature which is 80F. This will hopefully raised the soil temperature somewhere between 5-15C!

Monitoring the temperature

Basic Min/Max thermometers

We will be using multiple thermometers to record the minimum and maximum temperatures on the seed bed as well as other areas inside the polytunnel. When propagating seedlings we will use basic plastic propagation lids over the top to help create a nice humid environment inside.

These thermometers are detecting air temperature but the soil temperature is probably higher, I don’t have a gadget for that yet!

More than just a seed bed

One of my favourite Salvia varieties that does not set seed but can be propagated by cuttings.

The heated bed can be used for various projects such as overwintering established tender plants, as well as starting young seedlings in Autumn. It will also make an excellent cuttings bed and can provide excellent rooting conditions for many plants. I think we’ll even give Avocado a go!

We will be experimenting with all sorts of plants and learning how to make best use of this new growing area throughout the year. We like it so much we’re already considering building another one!

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