Just for fun my mother has started several in her kitchen. Four toothpicks stuck in the seed and suspended in a glass of water. One she potted up, grew for several years in her sun room. Only reached about three feet tall before it parished.

Eric

I haven't tried avocado, but there was a big avocado tree growing outside in the central FL neighborhood where I grew up, so at least some varieties are relatively cold tolerant. That one was a big, smooth skinned variety, but I never knew the cultivar name. Given the size of that tree, I think I'd research dwarf cultivars for the greenhouse.

As an aside, has anyone grown coffee plants in their greenhouse?