My (limited) experience is that you can graft any time of year but you may have to wait for a long time if you pick the wrong time... I remember doing some T-buds in august on an in-ground tree. Weeks later no movement, bud still green. Months later still no movement, bud still green. Come spring, TADA! bud starts pushing! I hope cleft on a stick in a pot works better in the "off season"... I'll be trying that with nordmann nagami this august when the CCPP has bud sticks available again...