Hoophouses and Greenhouses

The following information is provided by Dr. Carol Miles as part of her Vegetable Research Program.
 

Season Extension

With proper crop and variety selection, there are many vegetables that can be produced during the winter in Washington. In central and eastern Washington, cold protection is needed for most vegetable crops while in western Washington, hardy crops can be grown without cold protection while half-hardy crops do best with protection (Table 1). When you have selected a particular crop to grow during the winter, it will be necessary to then select a variety that is cold tolerant. Refer to seed catalogs for specific variety information.

Table 1. Classification of vegetable crops according to their adaptation to field temperatures (Source: Knott’s Handbook).

Cool-season Crops
Hardy   Half-hardy

Asparagus
Broad Bean
Broccoli
Brussels sprouts
Cabbage
Chive
Collards
Garlic
Asparagus
Horseradish
Kale
Kohlrabi
Leek

Mustard
Onion
Parsley
Pea
Radish
Rhubarb
Spinach
Turnip
Parsnip
Potato
Salsify

 

Beet
Carrot
Cauliflower
Celery
Chard

Chicory
Chinese cabbage
Globe artichoke
Endive
Lettuce

Photo of lettuce growing under fabric cloche
Figure 1. Lettuce grown during the winter under a fabric cloche.

Row Cover. The two most common materials used for cold protection are fabric rowcovers and clear plastic. Fabric rowcovers come in different weights and can increase day temperatures by 4-8° F. Lighter fabric rowcovers (0.45–0.55 oz./sq.yd) are generally promoted as insect barriers and are not recommended for winter use. Heavier fabric rowcovers (0.9–1.5 oz./sq.yd) are used for frost protection. Place rowcover over metal or PVC hoops to form a cloche (Figure 1). If you live in an area that receives freezing temperatures, do not place rowcover directly on top of plants as frost damage can occur where the fabric comes into contact with the plant. It is important to note that night temperatures under a cloche tend to be equal to outside temperatures.

Hoophouse. Unheated and unlit hoophouses can be an inexpensive means of extending the growing season throughout the winter. Hoophouses are meant to be 4-season structures and depending on the snow load and wind velocity in your area, they may need extra supports. When it comes to construction, it pays to read before you start to build.

Photo of Lettuce and Asian Greens growing in hoophouse
Figure 2. Lettuce and Asian greens growing in an unheated and unlit field hoophouse during the winter.

Unheated and unlit greenhouses are most effective in the fall and spring but with proper crop selection they can also produce a crop during the coldest months. Use greenhouse-grade plastic to construct your hoophouse for best light and temperature effects. Day temperatures under greenhouse grade clear plastic (6 ml) can be 10-20° F greater as compared to outside temperatures on a clear, sunny day however night temperatures tend to be equal to outside temperatures.

High Tunnels. Most high tunnels are designed as 3-season structures and are not meant to remain covered during snowfall events or during high winds.
 

Links/Resources

Hoophouse and High Tunnel

Hightunnels.org

Portable Field Hoophouse, WSU Extension Bulletin EB1825 (html)

The Hoophouse Handbook. 2003. Growing for Market. 60pp.
 

Greenhouse

The National Greenhouse Manufacturers Association

Greenhouse Resources for Small Scale Producers, WSU and OSU Small Farms Team