They can't realistically be shorn until the fleece is ready to come off. This isn't a time thing, it will be affected by many things including the weather, the sheep's condition and workload. There'll be a rule of thumb date for your area but it can vary by a couple of weeks or more depending on other factors.
People talk about 'the rise' and the fleece 'rising'. If you part the fleece you may be able to see a line, possibly yellowy, where the grease is rising up the fibres. It marks the line between last year's fleece and this year's new growth. That's the line they need to shear along, and it needs to be 1/4 to 1/2" from the body for them to get the clippers smoothly underneath it. Trying to shear before this has happened is hard work for the shearer and uncomfortable for the sheep.
Another sign is if the neck wool - at the front of the neck - is starting to peel away on its own. Once this is happening to a few of your sheep, they may be ready for shearing.
Your ewe with lambs may be ready a little later than your one without - the latter hasn't had any work to do so has been able to work harder on her fleece.
Dangers of sheep, especially barren sheep, who are hot and fully fleeced is that they get hot and itchy, roll and get stuck on their back. They can die if not found and righted.
Trees should be adequate shelter - so long as they can get out of the sun (or rain or wind
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