In article ,
Garrapata wrote:

I know a farmer who heated his cold frames using horse
and cow manure.

That's how it was done a century ago (and even now in "less developed"
countries. Somewhere I have some 'Garden Magazine' from 1904-1906 where
this is discussed. If I can find them I will try to scan the article.

We did this with our cold frames 30 years ago. Fresh Manure about 12
inches deep covered with growing soil. Then switched to electric cables
then stopped. Why has no satisfactory answer. I think child rearing and
shift work played a large part.

Bill

--
Garden in shade zone 5 S Jersey USA

No foreign intervention unless tyranny at home.