Ways of warming soil temperature and creating soil microclimates

Many of us struggle with spring frosts or colder summer temperatures, and would like to raise temperatures above, not only to avoid those frosts killing seedlings but to grow better tomatoes or fruit in south oriented walls.

Thus I have been spending the last few weeks measuring the temperature around my house, and seeing how is the temperature decreased or increase.

I wrote these mainly in º Celsius increases, since I am more familiar with these units, but if you think its Fahrenheit, it does not make a big difference.

This morning I had a morning low of 20ºF (-6ºC); the soil was frozen hard solid, because there is no snow cover to prevent it.
But I had a spot outdoors with the soil at 32ºF (0ºC), so it is remaining almost unfrozen and could be ready to plant much sooner than the surrounding soil.

Basically a black plastic sheet can protect provide a 3ºC warmer soil, while a peatmoss mulch can provide another 3ºC warmer soil. A small cold frame tent is warmer only around 1ºC to 2ºC during freezes. The one I have is nothing professional, it is more of a tent than a sturdy cold frame, so it does not warm much during freezes. But by combining laying a black plastic sheet, with peatmoss above it, under the tent, the soil beneath it is unfrozen, while outside is frozen pack solid. It is 6ºC warmer!

The effects of a simple black plastic covering the soil and a 10cm thick moss mulching is just a "miracle".

I also discover than generally sandy soil is around 2 to 3ºC warmer than peat or clay soil. So laying sand over the surface of the soil is another way of increasing the temperature in colder climates.

A south oriented wall, or a sheltered wall, will generally be 2ºC warmer during a freeze, so sometimes everything freezes hard, but not in those spots! The warmth that is stored in the wall thermal mass, is released during the night and increases significantly the temperature and the soil might remain unfrozen up to 2m away from that wall. It is quite a remarkable effect.

On a negative note, I live in a frost pocket valley, just 30 meters below the flatland surrounding it. My spot is around 3ºC colder during clear nights, and you can clearly feel the frosty cold just by walking the 2 minutes down the road into this small valley. It is also where the snow melts last and frost appears first.

So far we had the following variables:
- a cold frame might raise temperatures up to 2ºC
- a peatmoss mulch raises up to 3ºC
- a black plastic cover raises up 3ºC
- a sheltered spot or south oriented wall raises up to 2-3ºC
- sandy soil generally up to 3ºC warmer
- frost pockets can be up to 3ºC colder
- in a deciduous forest understory seems to be only 1ºC warmer during a freeze.

In summer I also measured temperatures raised by other methods, to allow me to be able to grow runner beans and tomatoes, in the cold Icelandic summer.

- a raised bed or growing in a black container can warm the soil up to an astonishing 5ºC (this is during sunny days, but obviously not during a freeze)
- rocks can warm just around 1ºC (a black plastic has a much warming impact)
- sun generally warms the air above soil around 5 ºC more than a similar shade location, during a freeze day, and up to 10-20ºC more during summer
- a south oriented wall can warm to up to 2-3ºC in winter nights, and up to 6ºC during sunny days (if its a metalic surface, it is probably even warmer)

These effects are not purely cumulative. But you can add them together to create a microclimate way warmer during winter freezes, and during summer days.

Please share your experiences with these too.

Therefore if you want to avoid spring frosts, grow your plants near to a south oriented wall, in a spot sheltered from frosty winds, and set up there a cold frame, with the inside soil layered with sandy soil above, and covered by a black plastic and peatmoss. This way the microclimate inside might remain warmer up to 8ºC.

If you want to create a really warm summer microclimate, set up a raised bed next to a south oriented wall (make this raised bed as raised as possible and by covering black plastic to further warm the soil. Sandy soil in its surface, and the south oriented wall can be metallic to further increase temperatures. Assure that the spot is protected from cold winds. If you set up a plastic cold frame over this soil, it will be even warmer! This way the microclimate will be much warmer than the surrounding, around 10ºC warmer than surrounding spots, even if it is just exposed to the air.

This is almost like increasing two zones, like from zone 6 to zone 8, or zone 9 to zone 11.

Ideally you would set up a huegelkultur next to a south oriented metallic wall, wind barriers, and covering with a simple greenhouse-like plastic cover cage around it, and black plastic covering the soil. In winter you do thick mulching, in summer you can reduce the mulch.

Good stuff Paulo , thanks for sharing.

This morning it was -9ºC and inside the coldframe it was -1ºC only (the temperature was laying on the soil, which was covered first with black plastic and then peatmoss on top. I had broad beans, peas, spring onions and brassicas (all seedlings) still alive beneath the plastic. The soil is not frozen. And during the day I remove the moss and the soil and plants under the plastic warm up to +13ºC, while outside temperature is just +2ºC.

I think I will add an extra protection around the plants. Between the plant and the black plastic sheet, I will enclose the plant with a cut water's plastic bottle, to ensure that the plant has space to grow and breathe. But with moss mulch around their stems, to protect from eventual freezing.

Very interesting thread Paulo, thanks for sharing.

Great post and observations, Paulo...the only thing I see that you've not addressed is wind. Do you not have a prevailing wind that needs a windbreak or diversion method...?

Ollie,

In calm clear nights, without apparent wind, the air flows both downwards and also coming from the north or east, where cold winds come. My garden is by a descending road that faces north, so its a kind of frost magnet, colder than the surroundings.

But by sheltering the wind (cold frame, shrub edge, a wall), one removes that cooling influence.

For instance, this morning it is -13ºC, but the soil under the cold frame + plastic sheet + moss cover is now only at -3ºC. It has been around zero for the last days. It is cooling much more slowly.
It is only partly frosty and frozen, not frozen solid hard as the surrounding soil. This helps a lot.

I added inverted plastic cups over each seedling, and under the plastic/moss cover. So they are really well protected and sheltered. They are peas, broad beans and spring onions. Not only they are undamaged, I can dig the soil if I want to, because it is not frozen hard.

I think that the moss is an excellent insulator, it cools much more slowly than other materials, and thus keeps the soil much less cold. The black plastic captures heat during the daylight. And the cold frame deflects the downwards frost and cold flow of air.


everything is growing a triple layer of black plastic, peatmoss and black plastic again, covering the soil, that remains unfrozen.


thick peatmoss is between two layers of black plastic sheets


under the entire thing, seedlings are further protected by inverted plastic cups


seedlings inside the mulch cover, are growing very well.


it is -13 C outdoors (around 8 F)


but it is only -3 C beneath the mulch cover (around 28 F, just slightly below freezing point)


soil beneath the mulch, is unfrozen


even further out, in a more exposed location, I set up a cold frame with the same mulch system inside


here it is the peatmoss inside the cold frame, that covers black plastic, which covers the seedlings


seedlings are also very much in good shape in there!

Check more photos in my blog post http://greenspot-english.blogspot.com/2013/03/how-to-plant-out-seedlings-in-deep.html

I am looking to build on some work done in the 90's in Alaska. They tested using carbon on snow and soil to increase the melting of snow and soil temperature.
Currently I am working with mulch to control weeds. But mulches and cover crops tend to delay soil warming by a few weeks.
I am thinking of using something like Radish as the cover this fall so I will not have heavy mulch in spring then adding course enough biochar so it will not wash off be get sucked into soil with freeze thaw cycle.
The combination of no mulch and biochar should help increase soil temperature to the point where root crop like potato can be added when soil is 42 to 45 degrees F.
I then can add a light mulch so soil can continue to heat. When soil reaches 60 degrees F I can apply heavy mulch to keep tubers cool and extend active growing season.

I would appreciate any comment on setting up a valid test for this.
I do not have any of the data from back in the 90's, I just remember that it did work.

It has now been 14 days since I planted several seedlings of broad beans and winter peas.
For 10 days, the temperature was a daily 10ºF (or -12ºC) and extremely windy - very harsh conditions even for peas and broad beans.

With protection, the soil hanged around freezing point for a few days, but the temperature eventually dropped down to 25ºF (-3ºC) and soil froze partially under the peatmoss and plastic. I was not expecting a miracle with such a prolonged freezing.

Nevertheless, about 80% of the broad beans survived. About 60% of the peas survived!

However next to the south oriented wall, which is also a sheltered spot, the survival percentages were much higher, because there I put more peatmoss.
I realized I can improve the change of survival by transplanting out smaller seedlings: 2-3cm rather than 5-10cm. And perhaps doing some hardening off for a few days, which I haven't done.


Therefore, after those 10 days of the big freeze, I moved much more seedlings still in their each individual tiny pot outdoors, under the coverage but not transplanted. Broad beans and peas again.
They have now been for 4 days with freezes during the night but above freezing during the day (23ºF to 40ºF) or (-5ºC to 4ºC). Soil is still frozen but of course not under the covers.

Of these recently moved plants: 100% of the broad beans are in good shape, and 95% of the pea seedlings are well.

I plant to transplant them after I see that they have been well adapted to the cold temperature. Unlike brassicas, it seems to be better to work with small transplant of legumes, because their stems become very fleshy and extremely prone to damage by freezes, when the seedlings grow past 3-5cm.

Paulo,
Have you ever tried adding a trench of actively working warm compost in a trench next to where the seedlings are going to supply additional heat.
I have in Alaska dug compost a trench as soon as soil can be worked and buried it then planted peas right next to or on top of trench to get some additional soil temp.

alex Keenan wrote:Paulo,
Have you ever tried adding a trench of actively working warm compost in a trench next to where the seedlings are going to supply additional heat.
I have in Alaska dug compost a trench as soon as soil can be worked and buried it then planted peas right next to or on top of trench to get some additional soil temp.

Yes I have tried

The peas I have told to transplant into a more sheltered spot, next to a south oriented wall, actually have sandy soil with burried ongoing compost. The soil is warmer there for many reasons: it is sand, it is sheltered and the buried compost.

Do you transplant larger pea seedlings or small ones?

If I wait to sown directly outdoors, I still need to wait a month. Soil is still somewhat frozen and seeds will take a long time to come out. But I will try them now under the protection and next to the buried compost.

I am not working in Alaska now but when I was I tended to use transplants were ever possible.
I would get them started then harden them off so they could survive the colder temperature.
One important part was how large I allowed the starts to grow. Each type of plant seemed to have a idea stage for best hardening off with minimal impact to overall growth.
Also each type of plant had an ideal soil temperature for transplanting. It did not pay to transplant below this temperature.
Some times I would do two or more sets of starts in order to catch the right soil temperature and seedling growth stage combination.

Question for you, why is the sand warmer?
Are you getting a cooling effect from water evaporation in clay or soil that can hold alot of water?
Or is it the poor insulation value of a soaked or wet soil due to water conducting heat?

I do know that in both Alaska and Ohio I have found that how the frost forms in the soil has an impact on what plant survive the winter.
There are a number of plants that I have to allowed the soil to become sort of dry in order to get higher survival rates.
For some plants it is lower moisture contend within the roots that helps keep freezing damage within the cells of the roots from becoming a problem.

I would be interested in how you control watering and soil moisture.

alex Keenan wrote:

I am looking to build on some work done in the 90's in Alaska. They tested using carbon on snow and soil to increase the melting of snow and soil temperature.
Currently I am working with mulch to control weeds. But mulches and cover crops tend to delay soil warming by a few weeks.
I am thinking of using something like Radish as the cover this fall so I will not have heavy mulch in spring then adding course enough biochar so it will not wash off be get sucked into soil with freeze thaw cycle.
The combination of no mulch and biochar should help increase soil temperature to the point where root crop like potato can be added when soil is 42 to 45 degrees F.
I then can add a light mulch so soil can continue to heat. When soil reaches 60 degrees F I can apply heavy mulch to keep tubers cool and extend active growing season.

I would appreciate any comment on setting up a valid test for this.
I do not have any of the data from back in the 90's, I just remember that it did work.

I've noticed how the grit / sand they put on roads in the winter is dark. Curious to see the effect on melting snow, I was surprised to see that the grit accumulates on the surface, accelerating melt initially, but then it seems to insulate the underlying snow. It reminds me of glacier morraine, where the ice gets covered with rock.

Anyway, it got me thinking about an alternative where the darker material doesn't accumulate on the surface like this. Maybe a dye sprayed on the snow? It may seem like an appalling idea at first, but what if the "dye" was made from macerated green grass or something? You could make it in the summer and freeze it. Then in the spring, you could thaw it out and spray it on the snow. As the snow melts, the green stuff will penetrate deeper into the pack.

Just a thought...

Put a lump of charcoal in a blender and mix on highest setting. Then strain using window screen or smaller size screen. This is then put in a sprayer and applied to snow when below zero so it will freeze.
This will melt your snow, just reapply at night when below freezing as snow melts.

If you used biochar as mulch, or mixed a good portion into the top layer, that in itself would heat the soil, wouldn't it? Any exposed biochar would be black, and extremely porous. Same deal as melting snow.

-CK

The problem here in Iceland is not really the snow. Our winters are very different than most places.

Our winter is an alternation of mostly rain, sleet and freezing rain, followed by hard freezes. Snow does not accumulate much, so the hard freezes come without protection to the soil. And the winds are very violent. Spring is very dry and sunny but also cold and windy. Freezes during morning, and above freezing during the day. Soil remains frozen for 9 months, but the top part is always melting and refreezing. Worst of all, in May, we often have one or two severe freezings just after some warm weather. The same erratic weather continues into summer, but always with cold weather ocasionally.

Therefore it is even more difficult to guess when to transplant. I can wait until mid June, the frost free season, but since air temperatures can be so mild during spring sunny days, I gamble in planting seedlings out, even if most of the soil is still frozen and nights are still freezing.

Last 2 years I planted brassicas, onions and peas in 1st May, they suffer some frost damage, but since April has similar weather to May, why not gamble with transplants already now and into early April, and then endure through ocasional freezes and almost daily frosts.

Alex: true, survival of plants does depend in moisture content. Often winter freezes here are too wet, because of so much rain before freezes. Plants do survive much better when planted in sandy soils and sheltered spots. But in spring, there is a lot of drying cold winds, and therefore plants can suffer, when daily temperatures are warmish but the soil remains frozen but dry. It seems sandy soils with top mulch for insulation is the best solution here. Also sandy soils provide much lighter structure than the native clayish and peaty compact soils.

Sounds like you need to protect the plants from the wind. Earth works / hugel beds etc.

Have toy considered using a used soda bottle with the bottom cut out as a cloche to protect the plant?

Paulo, Fascinating & informative post, thanks.

You mention that you put peat moss above black plastic in one setup and you put peat moss in between black plastic in another setup.

Question: Have you tried to just put peat moss directly on the ground then black plastic over it. And no peat moss on top of the plastic?

Putting peat moss on top of the plastic might actually cool down the soil. Since the moss will heat up in the sun, but that heat will quickly be lost to the chimney effect.

It's similar to layering blankets in bed, you want your non-porous blanket on top with all the fluffy porous blankets underneath in order to keep warm.

Another thought is have you tried using clear plastic over peat most. It might heat the soil faster by letting sun penetrate.

I don't have freezes where I am, but pools heat up much more with clear plastic covering than they do with dark color ones.

Great ideas Eric and Nick,

I have now plastic bottles as a "cloche" around the seedlings (to let the sun heat the soil and the seedling). In some seedlings, I also have peatmoss around the plastic bottle. This ensures extra insulation but of course it does not let the soil warm as fast.

At night I cover with black plastic and with peatmoss, if night is freezing rather than a mild frost. But I remove it around noon. I am still afraid of letting go of the peatmoss during the night and early morning because the temperature is quite freezing.

During day, I open everything and let the sun shine, if it is above freezing.

If its a below-freezing day, or otherwise cold winds, I let only the black plastic, so that it captures the heat of the sunlight. Except if it is a severe freezing, I let the double layer of plastic and peatmoss, to provide extra insulation, the whole day.

Everything seems fine so far (freezings have been mild lately), except that during severe freezings, the plants with sandy soil survive much better, because the soil freezes much less there. In the clay soils, many of the pea seedlings died. Too severe cold there and soil was moist and froze hard, even under the protection (even though the freeze was much milder over there).

Wind wise: I have plants either sheltered or under a cold frame tent. I plan during the summer to do a huegelbet to shelter further, but I am not sure if that is enough to shelter them. I think a shrub or tree line would be the best solution, but those takes 20 years just to reach a few meters high!

That's excellent news.

Remember to unscrew the bottle caps in the day to let the heat out, and screw them back on in the evening to keep the heat in

I updated how much does each method warms the soil, to prevent hard frosts:

- a plastic cold frame generally raise temperatures up to 2ºC, while a fleece only about 1ºC, however they prevent frost formation down to -3ºC (during the day the temperature might increase 15ºC inside a plastic cold frame, and 5ºC inside a fleece tunnel)
- a peatmoss mulch raises up to 3ºC and prevent minor frosts (but keeps soil colder during the day)
- a black plastic cover raises up to 2ºC and prevents minor frosts (but keeps soil colder during the day)
- a sheltered spot or south oriented wall raises up to 3ºC (warmest as you get nearer the wall or the sheltered spot)
- sandy soil generally up to 3ºC warmer
- frost pockets can be up to 3ºC colder
- in a deciduous forest understory seems to be only 1ºC warmer during a freeze. I have not measure how does the temperature is affected by a leafy forest cover
- I have a water line next to my house, and the temperature there is 2ºC warmer in freezing nights (and is facing north).

A combination of a plastic cold frame, with plastic and peatmoss covering the soil will keep the soil around freezing during hard freezes. If there is a night with -5ºC and day highs around 0ºC (the soil will be just around 0ºC). During a night freeze of -12ºC and day highs of -6ºC, the soil will be kept at -3ºC (just slightly frozen, but not solid hard). I was able to keep seedlings of broad beans and peas alive for a couple of weeks in these conditions.

Besides this:

- a raised bed or growing in a black container can warm the soil up to a significant 5ºC (except during winter when they will be colder because they are more exposed)
- rocks can warm just around 1ºC (a black plastic has a much warming impact)
- sun generally warms the air above soil around 5 ºC more than a similar shade location, during a freeze day, and up to 10-20ºC more during summer
- a south oriented wall can warm to up to 2-3ºC in winter nights, and up to 6ºC during warmer sunny days;
- if its a metallic surface, it is much warmer during a sunny mild day, but they will be probably colder during freezing temperatures

Please feel free to update your experiences with creating microclimates, and measurements of temperatures in those spots.