At last I have built the scarecrow.
There is a lot of conflicting and erroneous information on the forum on how to do this, so for those of you following me here is how to do it.
When you are ready to build it, i.e. you have crops growing, take an adept farmer not a master farmer as some said and put her (mine's a woman) on dry roots. Here is where to find them. At the top of the picture there are a lot of blue stones with a few white ones directly underneath and then underneath the white stones are the dry roots. Put your adept farmer on them where she finds sticks and she will start to build the scarecrow. Mine built half the scarecrow and then wandered off to do her own thing, so I put her on the roots a second time and she finished the scarecrow.
Easy when you know how. Unfortunately I was nearly out of food before I cracked it so I may have to start again.