Saturday, 5 November 2016

Flagstone or soil based greenhouse?

I have both a flagstone and a soil based greenhouse. 

The interesting thing about the flagstone greenhouse is that it retains heat for much longer than the soil based greenhouse. The flagstones seem to act as storage heaters cooling down slowly during the night. 

Soil based greenhouse with mulch of chippings

Slab based greenhouse

If your greenhouse is in full sun all day, allowing the slabs to heat up, then this is an advantage. My cold greenhouse (i.e. one without artificial heating) remains above freezing for most of the year.  I know that we have had mild winters recently and this might not continue but it indicates to me that there is an advantage, particularly when you don't use any other heating.  The slabs are on stone that was sieved from the rest of the allotment and this will add to the thermal mass of the slabs. 

Design behind the greenhouse.

There also might be an advantage due to reflected light from the slabs particularly as they are grey/white.  I think that this is a neglected feature that could benefit plants a lot. I keep the slabs as clean as I can to increase the light reflected and to reduce the possibility of disease.   However, washing the glass and the poly carbonate sheets regularly, allowing for maximum light to pass through them, is also very advantageous.   

Keeping the slabs clean with water has an additional benefit in the summer when the greenhouse needs cooling.  Putting water on the slabs will cool the greenhouse through evaporation.

Further, my soil based greenhouse is slowly sinking even though it is on an aluminium foundation.  I will have to remove the glass and polycarbonate from the greenhouse to straighten the foundations soon.  Maybe during the winter.   The slab one is not sinking.

Making holes in the slabs to screw the greenhouse down was very difficult, leading me to give up in the end. I am relying on the weight of the glass to stop it from toppling over in the wind. I have had it on slabs for over 30 years now (but not in the same place) and it has not even had a glass pane cracked by the wind. However, neither has the soil based one. 

In the soil based greenhouse I plant in pots or ring culture pots (with no bottoms) as I do in the slab based one. So no difference there.  However, I have a peach tree growing in the soil based greenhouse and I would not be able to do this in a slab based greenhouse, although you could have a vine planted outside the greenhouse and growing through a hole in the glass.  The peach is in a greenhouse to prevent it getting peach leaf curl, protect the blossom which comes early in the year and to accelerate ripening of the fruit.  If the greenhouse was bigger, I would plant an apricot and a nectarine in a greenhouse too and for the same reasons. 

I have a lot fewer weeds in the slab based greenhouse, although they find their way into the cracks between the slabs.  I have just cleaned the slab based greenhouse and only found one weed which was easily dealt with using a slab weeding tool. 

Although this is a bit of a chore, I replace the topsoil and the woody chippings in the soil based greenhouse every year,especially if I have been growing tomatoes.  It mitigates against disease and I  find that it is well worth doing. 

So I think it depends on what you want from your greenhouse.


  1. I appreciate your reminder of the thermal capacity of flagstones and similar materials. I too have two greenhouses, one soil and the other paved floor. My greenhouses get used for all manor of tender plants brought inside to overwinter.
    What in your opinion on 'plunge in soil' versus 'stand on stone' to protect frost sensitive roots?

    1. Hi Roger good to hear from you. Interestingly I have just been reading Abercrombe's "Every man his own gardener" where he suggests plunging pots of frost sensitive plants in tan or a mild hot bed. I plunge some of my frost sensitive bulbs in the peach greenhouse and they seem to survive even though I forget to water them.
      I have most of my frost sensitive plants in the slabbed greenhouse but I use the plastic covered, shelved, mini greenhouse to protect them and that means they are on a metal mesh. If I had any more I would keep them on the slabs and cover them in fleece to protect them.
      Really I don't like to keep frost sensitive plants in a cold greenhouse because I have lost so many doing this. I have given up on using paraffin heaters - I can never make them work efficiently and they cover the glass with a layer of soot. This is why I am investing in a conservatory where I can keep plants warm and look after them a little more regularly. I will also be able to set up a light box over a propagator - something I have been wanting to do for some time.

  2. I, too, have been experimenting with a greenhouse/coldframes to extend my seasons here in Texas.
    One extra bonus that I have noticed in my experimentation w/ FLAGSTONES is that summer squash/zucchini (cougette?)go BONKERS on those super hot flagstones in mid-summer. If you are a fan...expanding your experimentation in that direction may profit you as well.
    I just love trying new methods and ideas-I need all the help I can get, best of luck to you.