[Kleftiwallah]

 

Which veggie benifits most from bottom heat produced by a heater cable in the border of a greenhouse?

 When warming up the soil in an outside bed (using black plastic sheeting) and is it better to to have this in contact with the soil or a small air gap between soil and plastic sheet ? ?   
Cheers,   Tony.

[bravemurphy]

Personaly i dont think it matters wether the sheet is touching the soil or not it will warm it up.

Just make sure its well weighed down cos if the wind gets under it a bit could be a good few yards away and upset the person whoose plot it lands on :-)

[fatcat1955]

I would have thought the soil in a greenhouse border would be warm enough.

[sunshineband]

Even if you stretch the plastic tightly there will be an air gap anyway.

And it warms the soil just nicely 

[gremlin]

In my opinion plastic only warms the soil if it is already dry.  I once used  black sheeting on wet soil and when I removed it a few weeks later the soil was still cold and wet. The uncovered soil nearby had dried out by itself and was fit for sowing.

[Salmo]

See RHS advice

http://apps.rhs.org.uk/advicesearch/profile.aspx?pid=695

They say that black plastic should be in touch with the soil.

[Kleftiwallah]

Thanks to all, especially Salmo. 

As I've already streached black plastic on top of a metal mesh on one raised bed, I'll ensure the next raised bed I cover that the plastic is in contact with the soil and compare results when it's time to plant.