Greenhouses needn't be large or expensive. Charlie Nardozzi of the National Gardening Association shares some tips on smaller structures and the materials used in their construction. New building materials allow you to set up portable units in minutes.

One portable greenhouse kit opens like an accordion. After expanding it, the user installs a spreader bar, which fits into a specially designed bracket and extends along the roof line, to keep it taut. The entire setup and assembly takes less than three minutes, and the structure itself is inexpensive, usually costing less than $500. The plastic used in covering the structure admits 75 percent of the available light, which is perfect for seedlings, and resists ultraviolet rays. The plastic is tear-resistant and has zippers that permit the user to open the roof for ventilation. The plastic should last at least five years with average use.

The greenhouse may be collapsed and stored during the winter if desired. And if you'd like to extend the growing season in the fall, set it up over the tomato patch.

The greenhouse's frame is made of lightweight aluminum, so if used in a windy area, it's recommended that the structure be weighed down to secure it.

Shelves are available for this 6' greenhouse in the form of horizontal bars placed on the framework. Wooden slats are set across the bars.