[mdjlucan]

Can I put some farmyard manure at the base of them

[Aunt Sally]

What's the soil like where it's planted ?

[mdjlucan]

Clayish

[Aunt Sally]

Clay is not the best soil for lavinder as they really like chalky/alkaline soil, which is well draining.

I would rather add some grit to the surface of the soil and carefully fork it in a bit.  I don't think it will need the richness of  manure as it also likes poor to moderately fertile soil.