Can you plant trees on the top of hugel mounds? How big do the mounds need to be in order to plant trees?
I am planning to plant fruit trees along the back of my suburban (1/4 acre) property and wanted to create hugelkultur beds to do so. I want to plant them about 5 feet away from the fence and create a space to run some chickens between the fence and bed. As well as growing grapes and other vining plants up the fence.

Any help would be great appreciated.

Sure you can. There are things to watch for:

The tree roots need to be able to reach down to get to the water, so they may need irrigation for a while (and longer than if they were planted down the slope of the bed).

The roots need to get into stable ground to withstand wind. Probably not a problem if you are planting 2-3' tall trees, but may be if you buy 6'+ tall trees.

Thanks for the reply.
Is it better to plant them on one side instead of the top. I am not building a giant Sepp style mound so it's maybe more of a woody mound. Mine will be about 2-3 feet tall and 5 feet wide at the base. I am thinking of planting fruit trees on one side and berry bushes on the other.

I planted a hugel mound and used some old cottonwood branches and a few months later i had a cottonwood growing out of it. I haven't really ever watered it and it seems to be doing good, it has grown 3 feet in a year and it overwintered well. We are in the high desert and don't get much rainfall. The hugelmound is on the north side of one of the gardens so some water is probably making its way over there.

I would plant trees on the downhill side of your bed, that way the tree can get to the water easily. Plant things around the top of the mound that don't mind dry periods as it will dry out first. Once the bed has some ground cover on it and shades the earth, your bed will stay at a more constant moisture level. There are no real "rules" in the end, just try whatever ideas you come up with, try them. Hope that was of a little help.