Now is not the time of year for sowing, unless it's sowing an idea: As the weather turns colder and the landscape becomes washed in grays and browns, imagine a retreat, an oasis of lush greenery and brightly colored flowers suffused in warm, moist air.

A greenhouse.

Home or "hobby" greenhouses run the gamut from lavish, Victorian-style conservatories to primitive structures cobbled together from discarded window frames. Each creates its own oasis. Even my coldframe — nothing more than a large wooden box with a clear plastic cover — hints of the tropics each time its cover is opened.

For most of us, the greenhouse of our dreams would be a spacious conservatory, cozy for its abundance of lush, tropical greenery, with enough space among the 6-foot-long banana leaves and fragrant citrus blossoms to accommodate a small dining table and chairs.

But let's float back down to reality. That greenhouse is beyond many a gardener's budget, and it's hard in these environmentally aware times to justify heating a space enough to keep tropical plants happy in winter. Fuel needs rise dramatically with each degree you ratchet up greenhouse temperature.

Then again, newer greenhouse coverings, coupled with innovative methods of storing excess heat generated on sunny days, can go a long way to dampening heat losses.

Before letting your imagination run away with you though, consider whether you'd prefer your greenhouse to be freestanding or attached to your home.

A freestanding structure offers the most flexibility in design and siting, and is bathed in light from all four sides. Also, there's no need to integrate it with house design, or for it to look anything prettier than just functional.

An attached greenhouse requires more attention to style but it does have some advantages. Cozied up against your home, an attached greenhouse loses less heat. It can tap the heat-storing capacity of the home's wall where it is attached, especially if that wall is masonry, and can even tap into the home's heating system.

On sunny winter days, excess heat generated in the greenhouse can be vented into your home. That moist heat is a lot more comfortable than the dry heat of home heating systems, although some caution is needed against venting too much moisture into your home.

And getting back to that table and chairs that might be squeezed into your greenhouse: How likely are you to trudge across the snow with brunch to your freestanding greenhouse? If you have a wall facing in a direction that gets enough light for the plants you want to grow, the attached greenhouse is the one most likely to provide living space as well as functional space.

Deciding on the greenhouse's size and what plants it will house also can help determine heating needs. No need to get into other details just yet.

Let your imagination roam.