When is the best time to plant rhubarb and asparagus ?

what is your location??????

Both can be planted in late Spring when soil is damp, frost free and sunshine.

Yes, spring when last hard frost is done, is the best time because the roots get to stretch during the summer and fall. A very light frost will not bother the crowns in any way. The trick with asparagus is making sure all the long hair roots are pointing down! If you must, fall will also work but you do not want the plants growing and the crowns and roots need time to spread out and down. Tricky timing there. Enjoy your crops in a few years!!!

Depends where you live but in spring is best. Asparagus cannot be harvested for the first two years and you need to dig a ditch about a foot deep, put the plants long roots down, then cover an inch or so at a time as the plant emerges. Rhubarb is best in spring also. You cannot harvest rhubarb until at least the second year. It is necessary to let some stems on the plant in order to have the roots spread and the plant spread.