I would use a stake to mark the begonia's location and try to avoid planting bulbs directly under that space. I think if you leave a 4-6 inch diameter circle you will have room. Well, how big are the tubers? Will they be dormant or from pots? Accidentally cutting up the bulbs while planting the begonias is probably your biggest risk.

I hope your garden grows well. I have seen your diagrams I. Other posts and hope we get a picture if the final garden next spring. :)

Thanks for responding. I keep going back and forth on what I am goign to do with the garden as far as plants go (aside for the boxwoods) I think the bulbs are going to be a pain so Im not going to bother with them. At least that is the sentiment this week lol. Im thinking heuchera berry smoothie and lime marmalade would be nice alternatives to flowers for year round color but someone had brought up a good point about underplantings getting in the way of the spiral topiary maintenance. We'll see how it all ends up I guess lol. The boxwoods (fingers crossed) should go in next month. The other plants wont go in till sometime next year.