Building a Home Greenhouse

VK writes to us:

My husband is about to add a small greenhouse to the side of our house
here in a banana belt above Sonoma Valley. Do you have any
suggestions about what they need in a greenhouse where the temp very
rarely gets below 32? It will be made of double glazed glass doors.
Have you written about this in the newsletter & I have just missed
it? Thanks again.

Charlie responds:

Greenhouses are wonderful for helping tropical plants grow in non-tropical climates. I will be writing a longer article about this in some future newsletter but some important points to keep in mind are:

1. The gh (greenhouse) temperature will drop slowly until it matches the outdoor temperature at night unless there is a heater in the greenhouse. It still helps the plants since the temperature stays warm longer and is cold for a shorter amount of time but some source of heat will make a big difference to the temperature and the plants. The lean-to style gh you are going to have will gain some heat from being up against the house but I would still place another source of heat inside even if it is only used on nights that are close to or below freezing.

2. In the daytime, if there is direct sun on the gh, the temperatures can rise very high, too high for most plants. It is critical that ventilation be designed into the gh so that the excess heat on a sunny day can be allowed to escape the gh. There are several ways to do this, from automatic windows that pop open when the temperature reaches a certain level and close again when it drops below that level, to exhaust fans or just doors and windows that are opened and closed manually. Automated features are great and much appreciated once you start living with a gh on a daily basis. However it is done, a gh MUST have a way to get rid of excess heat or the plants will be cooked!

3. A source of water is very handy inside or very near the gh. Plants must be watered and the gh cleaned out from time to time so having easy access to water will save time and trouble later on.

4. The gh floor is important to ease of use as well. If it will be bare ground then the ground should be covered with weed barrier cloth. This will not only prevent weeds from growing and allow water to seep through but will also give you something to stand on other than dirt which can turn to mud easily in a gh situation. You can also put gravel down but it is still a good idea to cover the gravel with weed cloth since it is almost impossible to keep gravel clean and weeds will grow in it as well. Weed barrier cloth can be ordered online or found at garden centers. If the floor is going to be paved with some material like concrete then it is good to plan for water drainage when building. The water needs to go somewhere, down a drain or following a slope to some exit in or under the gh wall. You will want to wash the floor from time to time and it is convenient to let water from plant pots fall directly onto the floor so planning the floor so that it handles water will make the gh easier to use in the future.

5. There is more vertical space in most ghs than there is floor space. Since almost everyone fills up their ghs faster than they thought they would it is good to provide multiple places to put plants such as shelves or tables. They can even be hung from hooks!

6. In winter tropical plants like light but more important than light is warmth and in ghs warmth rises to the highest point. By placing plants off the floor they will be warmer. To get the best balance of temperatures an air circulating fan is helpful.

7. High humidity creates its own set of problems in a winter gh. High humidity and still air cause problems with fungus on plants. Good ventilation and air circulation inside the gh helps a lot. A fan that circulates air inside is standard equipment for the gh. It also helps to remove old flowers from the plants before the flowers start to rot and form mold which they will do if left in a high humidity winter gh.

Greenhouses are a super addition to gardening. They really help overwinter the plants and give them a much faster start in spring. However, they can fail miserably if the above points are not planned for and the gh is not built with them in mind.

Good luck, and please let us know what happens with your gh.

Anybody else want to build a greenhouse? We've been using them for 18 years and would be happy to share whatever we've learned about them with you. Just submit a question to this forum.

Charlie