Will bulbs in window boxes in z4a freeze?

I'm in Z4a and am planting bulbs for the first time this year. Most of them I've planted in the ground, but I'd like to put some tulip bulbs in my windows boxes to enjoy outside my window this coming Spring. My question is this: will they freeze in the window box? The dirt is just about 6 inches deep...but that means there's barely an inch of dirt underneath them. Is this a bad idea? Will they freeze?

Thanks!

I think this could be risky in your zone. I've tried bulbs in half whiskey barrels and they didn't make it. One option would be to plant your tulips in pots, either plant the pots and all in the ground, or cover them with mulch against a structure. Then in spring they can be popped right into your window boxes. That way they could be easily removed after blooming to be replaced with something for summer blooms. Best of luck!
Neal

I agree. I'm in Denver (zone 5b) and I've planted bulbs in containers for years. You need more protection than what a window box can provide. Gemini's suggestions are solid. I'm going to try bulbs in a large, 10" deep, 36" long self-watering planter as an experiment this year, but I'm not expecting much. The other bulbs I plant are in large, foam pots, and I put them up against the building (I live in an apartment building) for the winter. They usually do fine, but sometimes things don't come up. You could experiment, but I wouldn't expect much.

Thank you both for the help. We have another near-70 degree day today here in Minnesota (unbelievable!) so I'm taking a half day off work to get out there and finish up my projects....after I vote of course!

I've read the rule of thumb is to knock down the hardiness one zone if overwintering bulbs in containers. But, of course, it all depends on the size of the container. Here in 6a, I overwintered asiatic and LA lilies in 10 inch terra cotta pots and a terra cotta 20" by 14" planter last winter. None of the containers were sunk into the ground. The large one sat above the snow on a rotted log. Oddly enough, the potted lilies came up and 80% of the lilies in the dirt right next to them did not (I suspect critters might have gotten the ones in the dirt).

I didn't even think of it, but I've overwintered lilies in pots with no problem several times. I would have thought spring bulbs to be tougher, but seems lilies are more likely to tolerate overwintering in a pot. In zone 4, it would probably only be advisable with Asiatics, LA hybrids, trumpets and OTs; Orientals are usually listed as zone 5 hardy.