Lean to Greenhouse - Plant in pots or soil?

I am building a leanto greenhouse that is doubling as "double glazing" insulation and heat store for the house.

We have a house on wood piles with an open gap between the floor and the ground.  

In terms of the plant health I think it best to plant directly in the soil.


As far as heating, I am not sure.  On the one hand I am thinking if we plant in pots and keep the ground dry there will be less heat moving from the outside into the house area in the soil.  On the other hand, I think that watering the ground will help the day's heat transfer into the soil and make a large thermal mass battery.  
We live in a temperate zone.  The Grass level will often have frost to -4C in winter that melts by 10am, but the soil is usually at least 7C+ 10cm down, and 14C year round within 1m down.  There is likely going to be as much warmth coming up from the soil as frost coming in sideways.  

I have seen lean-to greenhouses in places that go to -20C that plant directly in the ground.  

Interesting question!

It sounds like the base soil temps are quite good.
Do you have any plans to insulate the soil inside the lean-to from the soil around it?

no, mainly because the only insulation I know of is rather toxic stryrofoam based.  I do plant to have double walls of plastic and raised beds and heat sinks at the edge to help buffer plants prom the cold at grass level.

It sounds like you have a good plan so far. More to William's point: try to insulate that exterior ground with leaves, old hay, or straw atop a double layer of newspaper. No toxic ick factor and possibly better results

Hi Annie,
I wouldn't be too worried about planting in the ground vs pots when it comes to heat. The soil doesn't like to change temperatures very fast. The air temp is going to be what affects the heat in your house much more than the soil temp.

I don't think you'll like the results if the perimeter of the greenhouse is not insulated from the outside soil, you're essentially creating a giant cold frame, yes it gets warm during sunlight hours but it cools just as quickly out of them.  That -4C (25F ?) in winter will kill everything the least bit temp sensitive, no matter how quick the sun comes out. I'd remove the soil the yard or so to the 14C (57+ F) point from the base area, put in a thermal break at the perimeter, even if its wood, recycle the removed soil into pots for plants that can be moved about to their particular needs, you also can quickly remove something that a problem from aphids to blight, without losing everything.  that starting point being higher could allow you to maybe push some of the excess  heat under the house to warm the floor if blocked on the other three sides

Since you are using plastic glazing , I suggest layering plastic and borax sprayed cardboard on the floor of the greenhouse, and building your raised beds on top of that.

Snake a length of corrugated spotted drainpipe through the base of each bed, and use a fan to move air through them during the warmth of the day.
This would make each bed a small Ground Air Heat Transfer battery.

You can plant directly in the raised bed, or in containers nestled into the raised bed medium, or just put containers on top of the raised bed.
Because there won't be any snow, wind or rain inside the lean to, another layer of plastic can go over the raised bed, with minimal support needed.
Each layer of glazing is supposed to change the environment inside it to match one USDA  Zone warmer than the actual location.
So inside a low tunnel located in a  greenhouse on a zone 6 property the environment could be like a zone 8.

As for being a heat battery for the house,  I think the lean to will be more like a buffer against low temperatures and wind.
You mentioned the house is on piles, how high up are the piles?
Does the house have insulation in the walls, ceiling and floors?

The house in on piles that are about 40cm off the ground.  IT does have insulation in ceiling, under house and at least some external walls.  ((Definitely in the new section walls build 20ish years ago, not sure about the walls built 40ish years ago).

One idea for Annie could help overall warmth for the house. Since cold winds will still be blowing under the house, even with a greenhouse,  try to block off the other sides. A fast way is seasonally stacking straw bales or bagged leaves; a better all-season solution would involve metal of some sort, like 18 inch flashing. In a pinch I have used 20 inch strips of plastic stapled on top and anchored to the soil with rocks.

With this accomplished,  your lean to will stay warmer as well