As a general rule, I would always do the last clip no later than the end of January - so should be fine. However, I know others who have clipped later & had no problem with the summer coat coming through properly.
its only mid jan!!!!!!!!!!!!!!!! get clipping! you can clip til end of march ! for the clipped look! you can clip all summer for health/fitness reasons! and no it will not spoil your horses coat!
I'm clipping one tomorrow (first clip) and will be taking everything off the other one once the weather warms up probably end of Feb, maybe even March, then his coat all comes back at the same length
I think you' ve got at least til the end of Feb.
I clip throughout the year with my heavy cob as he gets so hot when unfit, and plan to do him next week, if I can start getting him back into work again after all this awful weather.
Stinky was clipped on 2nd Jan - just in time for the snow and probably will now be fine as he was done with the A2F blades.
Farra is going to have her blanket converted to full clip 2nd/3rd week of February, ditto Jet the welsh pony so they have nice coats for April when they start shows.
As long as you clip before they moult you will have a good summer coat, but if you don't show - clip away at any time. I used to clip Cairo and Jemima all year round, but Farra and Stinky don't have such harsh thick coats so are ok in summer (so far).