I cover as soon as I sow. Got fed up with fleece and anything fiddly to remove and put back, so I made my own easily removable tunnels with hoops and mesh - bought the hoops and a bit more mesh than I needed, cut out and hemmed two "ends" using a hoop as a template and stitched them to the main fabric. Sow the carrots, put the hoops along the row, fit the tunnel on top and peg it down, easily watered and easy to pull a few carrots and put back. Got the idea when I saw a similar piece of kit in a catalogue at a horrendous price.