It is very useful to have some extra warmth
when sowing seeds and growing young plants in the spring.
This is really important with long season heat loving plants such as tomatoes,
especially with the cooler growing season that we have here in Ireland.
I have found that it is beneficial to give the tomato plants an early start to the growing season
and to get them to grow reasonably big before transplanting into the unheated polytunnels,
so that we can get an earlier and larger crop. But this generally requires an extra growing
space to provide this additional warmth and light, and over the years I have used several different
methods to provide this better growing conditions for tomatoes and all the other warm season plants.
This season I built a compost heated hotbed for a large batch of tomatoes to grow in while
there was still a risk of frost in the spring. But this attempt failed, and almost killed all
of the plants, but not for the reason that I would have originally thought.
Like many people when starting out, I used to grow these plants in a sunny windowsill.
But even with and extra lamp the young plants often became long and leggy, which made them more
difficult to manage, and as I started to grew more and more plants, even this option wasn’t enough.
I built a mini greenhouse in our back garden out of scraps of polytunnel plastic, which provided
a much lager space that got a lot more natural sunlight, and with a soil warming cable buried in
the sand base I was able to raise the temperature enough to enable the plants to grow quickly.
But this was at the expense of a lot of electricity, and with the availability
of highly efficient LED lights, I also built a germination light stand in the house, providing
sufficient light to the already warm space. This combination has worked really well for
the most part, and has allowed me to grow an increasing number of plants,
but the electricity use of the lights and soil warming cable definitely added up.
Earlier this year, I needed additional heated growing space for the big batch
of 150 tomato plants that I was going to transplant into the new large polytunnel.
The plants were getting too big for the mini greenhouse with the soil warming cable,
and I had lots of other plants coming on that needed that space, but I felt that
the weather was still too cold to transplant them into the growing beds of the polytunnel.
So I decided to use a much older method of using the heat of a compost pile
to create a temporary hotbed in the polytunnel. The plan was to use this temporary structure to
provide with enough warmth, light and space for the plants to be able to grow for another few
weeks and to protect them from the overnight freezing conditions that were forecast.
The heat from the decomposition process in a compost pile can be very useful,
if managed well, and appropriate materials are used.
Apparently decomposing horse manure was often used to provide heated growing conditions, before the
convenience of electricity, and when abundant amounts of horse manure was widely available.
I know a few people who have filled a simple bin with fresh horse manure and other materials,
and then set the plants directly on top, so that the rising heat from the decomposing material
would provide gentle warmth for the plants above. I don’t have easy access to fresh horse manure,
and most growers don’t, and I wanted to try using this method with the abundant materials
that I do have access to, especially grass. Last summer I had built a temporary shower for
an off grid camping area, with the water heated by a container filled with decomposing grasses,
and it worked quite well. And I thought I would be able
to use a similar process to keep the tomato plants in good condition for a few weeks.
I built a base out of cut off pallets in the corner of the new polytunnel,
on top of the ground cover fabric that was keeping the weeds suppressed.
This produced a bin 3.6 meters long, 1.2 meters wide, and about 40-50cm deep,
which would hold almost 2 cubic meters of material, which I figured was enough
to build up substantial heat, but I also didn’t want it to get too hot.
I filled the bin with grass clippings from the first cut of the season of a large lawn area,
mixed with straw and other materials, and watered everything really well.
I placed a few cross pieces of wood to support a wire mesh for the trays of potted plants to
be placed on, and screwed pipe hoops to the sides to be able to cover everything with horticultural
fleece to keep in some warmth at night. The next day the material started to heat up,
though it wasn’t too hot, which was fine as I only wanted a little bit of bottom heat,
and I spread the 150 large tomato plants out on trays above the compost.
The plants were in decent shape, though a bit tall from the constrained conditions
of the growing space hearted by the soil warming cable, and needing to be planted reasonably soon.
I was hoping to have a balance of enough warmth overnight to keep the plants from freezing,
but not so much warmth that the plants would grow too quickly.
Shortly after I moved the tomatoes into their new space,
we have a few nights where the temperature dropped below freezing for most of the night.
These hard frosts can be quite a bit more damaging that the light grass frost that
we more frequently get, and can even damage the tender plants inside the polytunnels.
I put two layers of light gauge horticultural fleece over the hoops,
and with the temperature of the compost about 30ºC I figured this was more than enough protection,
especially as it was inside the polytunnel. The next day most things seemed fine but I
noticed a few leaves on several plants were not looking good.
It didn’t look like frost damage, but I wasn’t sure what else it could be,
and with even colder temperatures forecast for the next night, I covered the plants and
compost with a few more layers, just in case. I also set up temperature logger to check the
effect of this covering over the moderately warm compost pile, with one sensor outside, one in the
polytunnel, one in amongst the tomato leaves, and one buried in the decomposing material underneath.
The next night the temperature went well below freezing for most of the night,
with very little difference between the temperatures inside and outside the polytunnel.
But the leaves of the tomato plants over the compost and under the additional fleece protection
remained well above freezing over night, and during day the temperatures were not high enough
to cause these heat loving plants any stress. The following night wasn’t quite as cold,
but the fourth night was even colder, with temperatures dropping below minus 4ºC.
During this time I had noticed the leaves of the plants getting worse,
but didn’t know what was causing this and didn’t take time to figure it out
and to shift to some other method of keeping the plants warm enough overnight.
A few days later the plants were in really bad shape, with most of the leaves
discoloured and wilting, and I suspected I might have killed all of the plants.
The temperature logger showed that it never got cold enough to cause frost damage,
or too hot during the day which was the other possibility with the compost below.
I did notice that some of the leaves were in better shape closer to the corners, where air
could get in, so this led me think of only option was something in the air surrounding the plants,
with the multiple layers of fleece preventing fresh air getting in.
As the plants were effectively enclosed in with the compost, the active decomposition could have
been using up all of the oxygen in the air, and the the multiple layers of fleece could have
been slowing down more oxygen getting in. Plants usually give off more oxygen than
they need during the day, but apparently at night, when they grow they need extra oxygen,
and if the biological processes within the compost was using up all of the oxygen in the air,
there might not be enough for the plants to grow, especially after 4 nights in a row.
The compost would be giving off a lot of carbon dioxide, which I don’t suspect is a problem, and
I don’t know if methane would cause this, but I do suspect that ammonia might have been an issue.
I didn’t know this at the time, but apparently fresh decomposing
material can give off a lot of ammonia, and it is possible that the leaves of
the tomato plants were poisoned by too much ammonia or some gasses that were
released off from the compost below, and held in by the multiple layers of fleece.
Whether it was not enough oxygen or too much ammonia, or some combination of the two,
it seems likely that the lack of ventilation over the compost was the issue,
and I had basically smothered the plants trying to protect them from the frost.
Thankfully there was still some immature green leaves left on the plants,
which continued to grow over the following few days, as the older smothered leaves died back.
I decided to delay transplanting for another bit, to give the stressed plants a chance to recover,
but unfortunately the following week another two nights of hard frosts were forecast,
and I needed to protect the plants again. I decided to rely on the thermal mass of the soil,
under a covering of fleece to protect the plants, which is a method I have found to be effective in
the past, especially when I included containers of water to provide even more thermal mass.
During previous cold nights I had used a thermal camera to try to better understand
what was going on in the polytunnnel, and noticed that the large grow bags
of potato plants which were in the same polytunnel were quite warm.
So I set half the tomato plants directly on the soil, in between containers of water,
and set the other half of the plants in between the large grow bags,
with the whole row covered by two layers of heavy gauge horticultural fleece held up by pipe hoops.
I set up the temperature logger again, and the temperature in the tunnel dropped below freezing
for 7 hours overnight, down below -4ºC again, but the plants remained above freezing in both setups.
The thermal mass of the soil, water containers and bags of growing medium
gently released heat over the full night and two layers of fleece kept the temperatures
between 6-8º above the air temperature inside the polytunnel which was more than enough.
Thankfully that was the last hard frost we had this spring, and all of the plants continued
to grow, though they were set back quite a bit. By the time I transplanted them into the final
growing space of the polytunnel, I felt that they were behind the other batch of tomato plants
that I was growing for the smaller polytunnel, even though they were sown three weeks earlier.
So I figured the smothering had probably set the plants back by about four weeks.
But today, two months after they had been damaged, they are all very strong and healthy plants and
already setting quite a lot of fruit, so perhaps the setback wasn’t quite so drastic after all.
I was really lucky to not lose all of these plants,
but this is one of the key issues with starting tender plants so early,
and growing them in warm enough conditions so that they grow quite quickly.
I hadn’t expanded the heated propagation space enough to accommodate the larger
number of plants I was growing for the new large polytunnel this year, and I did not want them to
grow too tall in the crowded space. And we unfortunately had the coldest
nights of the spring just when the plants needed additional space.
In hindsight perhaps I should have just transplanted these plants into the ground
and covered them with fleece, as this would have saved me a lot of work and hassle,
but I think that I still would have needed at least two layers of fleece and a lot of containers
of water to protect them from the frost. On the other hand, if I had made the compost
out of material that was able to get a lot hotter, it might have produced enough heat
to protect the plants without being covered, but I am not so sure I would have trusted that,
and I probably still would have covered them with fleece on the really cold nights.
And with a much more active compost pile there probably would have been a lot more gasses
produced which might have still caused problems. This is all something that I want to explore
some more, to better understand what mix of materials would produce good growing conditions,
for a long enough period of time, and how long to wait after mixing the materials before placing
the plants over it without causing problems. I am interested in exploring different ways
of separating the active compost pile from the plants, while still getting the benefit
of the heat, and perhaps a simple sheet of plastic underneath the plants might be enough.
Or I could use a water pipe and a small pump that would be able to transfer heat
from the centre of the compost pile to a bed of sand under the plants.
All this starts to get a little bit complicated and a fair amount of work building a system,
gathering all the materials, and mixing all of the compost, especially compared to the
convenience of a permanent setup with a soil warming cable that can simply be plugged in.
But I think it can be really useful and energy efficient to use the heat of decomposition to
help with plant propagation, and there are a lot of different possibilities to explore,
I just want to make sure I don’t suffocate the plants again while trying to protect them.