A Greenhouse is not always a hothouse as many new gardeners believe. Sometimes it is a cool house with a low temperature that is ideal for growing many kinds of hardy bulbs, annuals, perennials, and alpine plants.
Pit Greenhouse Relies On The Sun
The pit greenhouse is in this low-temperature range for it relies on on the sun and earth for warmth, needing additional heat only during exceptionally cold spells or in the far North.
Construction of a pit greenhouse is simple and can be done by anyone handy with a shovel, a saw, and a hammer.
Selection of Sunny Site
First of all, select a sunny site, preferably on a south slope or against the south side of a building. See that nearby trees and buildings do not cut off the sun at any time of the day.
Drainage should be exceptionally good in the spot chosen. Sandy soil usually drains well, as does sloping ground, but clay or hardpan would need some drain tile or other means of drainage so water will not collect on the floor of the pithouse.
Pithouse Foundation
The pithouse foundation goes below ground. Mark out an area equal to the foundation size of the greenhouse and dig out all the soil to a 4-ft depth. Any size greenhouse can be used but the depth below ground level must be at least 4 ft.
The foundation is made simply by laying walls of cinder block without mortar. A little water may seep through the blocks but this will help keep up the humidity of the pithouse.
Covering of Floor is Not Allowed
The floor should not be covered since the warmth will come up from the earth. A lath walk or gravel path can be placed in the aisle between the benches for better footing.
If the pithouse is built against a dwelling, an entrance can usually be made from the house’s basement.
For a free-standing pithouse, an additional excavation is necessary for a flight of steps leading down to a door at one end, if you do not want to use up space inside the greenhouse for this.
Prefabricated Glass for Foundation Walls
A prefabricated glass or plastic greenhouse can be set on the foundation walls, or the superstructure can be constructed with 2 x 4’s.
Glass or plastic should be used to cover the south side, but since the north side and the two ends will receive no sun.
It is best to cover them with asphalt shingles, boards, or similar opaque material, and line them with some insulating material covered on the bottom side with light sheet aluminum. The aluminum not only insulates but reflects light that enters the south roof.
Placing of Bench
A bench can be placed against both walls and in the center—depending upon the width of the house, or plants can be set right into the ground.
For best light and headroom, the top of the benches should be about 10” inches below the outside ground level. This is also a comfortable working height.
The ground bed can be outlined with boards, bricks, or cinder blocks to hold soil 8” to 10” inches above the floor. But this is not necessary if you prefer to have the beds level with the walk.
Insulation for Retaining Interior Heat
Straw mats, rugs, or other insulation will be needed over the plastic or glass roof to retain interior heat, whenever there is no sun and the temperature drops to 25°.
The covering is always applied in the late afternoon before the sun goes down, or on stormy days when the sun does not appear. The plants will not suffer if the covering must be left on for several days.
Some gardeners install fluorescent light for such an emergency. Enough heat from the sun and the ground will be retained to maintain a temperature of about 40° even though the temperature drops outside.
Connecting Pithouse With A Heated Basement
If the pithouse connects with a heated basement, the door can be opened on very cold nights or an electric heater can be used for growing more tender plants.
Pithouses have come through unscathed by low temperatures even during cold New England winters.
Choosing of Plants for Pithouse
The important points are to choose plants that like a cool climate and to remember to roll down the insulating blanket early enough in the afternoon to retain the trapped heat.
During the cold dull days of midwinter, blossoms will last a long time and plants will show little signs of growth. After the turn of the year as the days lengthen, plants will spurt into growth.
Some of the plants that thrive in a pithouse are;
- azaleas
- calendulas
- camellias
- carnations
- chrysanthemums
- daphnes
- hibiscus
- Phlox divaricata
- pansies
- primroses
- roses
- snapdragons
… spring-flowering bulbs —like daffodils, hyacinths, and tulips as well as the little botanical species, and many other kinds.
Pithouse Gardening
Pithouse gardening is economical since it saves on a big item in greenhouse maintenance—fuel.
But, you must be on hand every day to put on the covering while the last rays of the sun are still shining on the roof. The reward will be refreshing spring bloom all winter long.
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