What Kind of Greenhouse to build

Q.  Are greenhouses really worthwhile?  And what kind should we build – quanset/tunnel, PVC, steel rebar, or wood, and what about heating and cooling?

A.  Greenhouses are an excellent way to give you between 4 and 8 weeks longer harvest period in your garden, as well as minimizing problems with pests and diseases, by allowing you to have strong, healthy and robust plants ready to place in your garden by the time others are just planting seeds.

Please consider three greenhouse issues Dr. Mittleider has dealt with – and the way he recommends they be handled.  While every family’s situation may be unique, there are several common elements, and we will discuss how to avoid three common problems.

In Dr. M’s book called Grow-Box Gardens (AKA More Food From Your Garden), chapter 10 is devoted to building small greenhouses, and the plans are quite thorough.  Go to the Store on the website, click on Grow-Box Gardens, and you can download and read the chapter on building a small Mittleider-recommended family garden-size greenhouse.  It has a strong wooden frame, supplemented with PVC, and that is the first issue I’d like to address.

We recommend you build your greenhouse to last, rather than have the first heavy snow load break it down, and so would recommend a wood or steel frame – at least for your main structure.

The second issue is heating/cooling and air flow, which is almost as important as the first.  Unless your quanset or tunnel-type greenhouse can have the sides easily rolled up, you will likely have problems with cooling in the summer months, because electric fans are expensive, and not always even available in gardens. 

The “Continuous Ventilator” running the length of the roofline on the current Mittleider-recommended greenhouse plan (Chapter 10 in Grow-Bed Gardening – free download in Store section) is the best solution, in our experience, since heat rises, and venting out the top eliminates the hottest air while drawing in cooler outside air from the sides and ends.  On large structures and in hot climates we also have at least one side able to roll up from table height to the roof line, thus increasing the venting to the maximum.  If flying insects are a problem we recommend screen be placed on all vent areas.

Thirdly, we recommend you make your greenhouse tall enough so that you can have all your plants off the ground on tables.  Some tunnel/quanset plans are very low, and plants are on or in the ground.  Using tables will help avoid problems with pests and diseases, which will spread like wild-fire in a greenhouse if they get started.

To summarize, I recommend the “continuous ventilator” wood structure design as my first choice, and as a second choice I would recommend the plan in chapter 10 of Grow-Box Gardens – making sure you allow for rolling up the sides for continuous ventilation.