The mini-greenhouse is a season extender. It is larger than a cold frame and shares characteristics with it and with hot beds, depending upon the type of greenhouse. Mini-greenhouses can range in size from a uniquely glazed window to a freestanding building that features transparent materials over a sturdy, but minimalist framework. Each greenhouse type has its unique strengths and weaknesses as a growing location. As is the case with many gardening projects, the trick is to balance the needs of the plants, your growing season and your budget against both your pleasure in your greenhouse and the products that can be grown there. Greenhouses can be used as a place to start garden plants, a protective environment for tender plants – both decorative and useful, a year-round growing facility or even as a solarium – a comfortable place for humans to relax and enjoy sunshine or shade.

The Bay Window Greenhouse

A bay window is an excellent place to locate your first mini-greenhouse efforts, especially if your home already has one. A word of warning: they are moderately expensive. A new window from Lowes Hardware will run a little over $600.00. However, they will add lasting beauty to your home and they are easy to convert to a growing space. They do have one drawback. Unless the window includes a glazed ceiling, it will share the difficulty encountered in many window gardens: a lack of sunlight. But it can be outfitted with a grow light in the ceiling. Add fitted metal shelving to avoid shadows, and your bay window becomes a mini-greenhouse. It is a superb place for tender herbs, house plants and even those baby garden seedlings.

The Protected Garden Shelf Unit

A less expensive semi-outdoor/indoor “greenhouse” is the metal greenhouse shelving unit that is enclosed in a plastic cover. A good unit can be purchased from a variety of sources such as Amazon, Lowes, or a garden nursery catalog for around $40.00 or less. These can be placed against a south-facing wall outside in early spring as a place where plants can be slowly acclimated to the outdoor environment. Used in this way, they are similar to a cold frame. However, they can be given a more stable temperature by using heat cables to warm the growing trays. Placed against a wall, they also benefit from the wall being warmed by your home heating. One Amazon reviewer noted that while they are not very sturdy for outdoor use, they make an excellent protective environment indoors. She raised African Violets, and stated that these units made it easy to maintain the humidity that these plants love.

The Lean-to Greenhouse

These range from structures that are scarcely larger than the garden shelf unit to full-sized rooms that are attached to your house. They can be temporary units or permanent ones. Ideally, they will be located on a southern exposure. The lean-to greenhouse benefits from your home heating, and it can return some of that heat during the day as passive solar heat. With careful planning and shade solutions, they can remain a pleasant space during the warm months of the year. However, being attached to the house means that one side of the structure does not receive sunlight. Some owners use that side as a Trombe wall – a wall specifically structured to be a heat exchanger. Others use that area to install fish tanks or similar aqua culture. Your lean-to greenhouse can be completely dependent on passive solar and shared heat from your home, or it can have its own heater. One thing it must have is ventilation. Even on cold winter days, sunlight on glass or plastic raises the temperature within the greenhouse. Furthermore, ventilation allows for an air exchange and creates the ability to regulate humidity.

Free-Standing Greenhouses

A free-standing greenhouse is one that is independent of other buildings. Your greenhouse could cost as little as $125 or it could cost several thousand dollars. There are so many commercially made greenhouses and ways to build your own greenhouse, that it is really hard to put a specific dollar value on these. There are unheated season-extender minis as small as five foot by four feet, that are scarcely more than a film of plastic stretched over a tubular framework. With good tie-downs (essential in spring storms), placed on the sheltered side of a larger building, these little guys are great for those seed flats that are overflowing your living space. Of course, there are larger greenhouses that still qualify as “mini”. Perhaps the king of these (or queen) is one made of polycarbonate panels over an aluminum frame. The double-sided panels have little internal braces – all made of polycarbonate – that capture sunlight and diffuse it, helping to eliminate dark spots in your greenhouse. With the aluminum framework, these structures are reasonably sturdy and will last through many seasons – unlike some of their less durable cousins.

Greenhouse Furnishings

While the external structure of a greenhouse is important, so is the internal furnishing. Some greenhouses are sufficiently sturdy that plants can be hung from the ceiling supports – others are not. Wire work benches and shelving helps to limit shading that can occur when using boards to support your plants. Plants need water; therefore, it is also a good idea to have a plan to provide water in your greenhouse. This plan can range from an old-fashioned watering can to running water that is piped in and filtered. Some greenhouses use a container system that helps allow unwanted additives in the water to settle out before using it on your plants. If you plan to use your greenhouse as a place to relax as well as grow plants, a table, comfortable chairs and perhaps even a lounge could be nice.

Greenhouse heating

Your greenhouse can function using solar heat, heat produced by a compost heap, or it can use additional heat from an electric, gas or oil heater. The different types of greenhouse heaters are too numerous to list here, but it is important to use a heater that is intended for this type of space. First, some types of oil, gas or even wood heaters create fumes that can damage some types of plants. Second, not all electric heaters or heaters with electrical parts will hold up to the humidity in a greenhouse environment.

Greenhouse Ventilation

Ventilation is key to good plant health in your greenhouse. Even if it is set up for passive solar heating, a warm day in mid-winter can wilt your plants in a hurry. If you are heating with gas or oil, it is even more vital. Thermostatically controlled vent flaps can save you a lot of work and worry by opening and closing the vents automatically as needed. Ventilation also helps prevent mold, mildew, fungus and certain insect infestations in your greenhouse.

Greenhouse Summer Cooling

In summer, ventilation and some sort of shade is vital on the warmest days of summer. Polycarbonate glazing can help with this by diffusing summer sunlight and cutting down on glare. Removable panels that can have shade fabric added instead of glazing is also useful, as are operable windows in addition to ventilation flaps.