Growing Vegetables in a Hobby Greenhouse

Outline

• Extending the growing season
• Passive solar greenhouse
• Cool season vegetables
• Warm season vegetables

Extending the Growing Season

Off-season vegetable production in the hobby solar greenhouse is an enjoyable way for year-round gardeners to extend the harvest season of fresh vegetables.  However, without the expense of a greenhouse, gardener can extend the growing season weeks to even months with cold frames and plastic tunnel gardening.  For details on frost protect and cold frames, refer the CMG GardenNotes #722, Frost Protect and Extending the Growing Season.

Winter vegetable production in a greenhouse is only cost effective with an energy efficient greenhouse structure, a well-designed solar collector, and optimum management. 

Winter vegetables have a slow growth rate due to low light intensity.  Crops should be planted to obtain a near harvestable size by mid-October.  The use of artificial light for vegetable production (except for starting transplants) is generally not cost effective.

A gardener’s success is dependent on the greenhouse design and construction to conserve energy and on the management care given the greenhouse crops.

Before investing in a greenhouse, carefully consider your real interests in extending the gardening season.  Are you only interested in adding a few weeks to the harvest season?  Are you interested in year-round gardening in a solar greenhouse OR do you need a winter break from gardening activities?

Passive Solar Greenhouse

For the gardener considering a passive solar hobby greenhouse, here are a few key points to consider.  Refer to other greenhouse references for additional details.

For solar collectors, any area with direct sun, but not blocking solar illumination of plants, is a potential location.  For a hobby greenhouse, solar collectors are typically built into an insulated north wall.

A solid brick wall on the north makes a good solar collector.  Brick absorbs 30 to 35% of the solar radiation.  With a brick storage wall, the greenhouse quickly heats on a sunny winter day and ventilation will be needed by mid morning.  [Figure 1]

Figure 1.  Brick storage wall in passive solar hobby greenhouse – Thermal storage mass is a wall made with two layers of brick filled with concrete.  In this well-built structure, nighttime temperatures dropped to 35°F with no supplemental heat when outside temperatures dropped to –17°F.  Note young crops in raised-bed style garden with drip irrigation.

Water storage using plastic milk jugs makes a great storage system.  Water jugs absorb 90% of the solar radiation, holding three times more heat than brick or rock.  This increased heat storage holds night temperature higher longer into the night, resulting in slightly improved crop growth compared to brick storage.  [Figure 2]

Figure 2.  Milk jug water storage wall in a passive solar hobby greenhouse. Disposable milk jugs on left and returnable milk jugs on right are spray painted flat black.  In this well-built structure, nighttime temperatures dropped to 39°F with no supplemental heat when outside temperatures dropped to -17°F.

With milk jug storage, spray the milk jugs with flat black paint, and add one tablespoon of Clorox-type bleach per jug (to prevent algae growth in the warm water).  Secure the cap back on the jug with a ring of caulk.  Place the milk jug on a bookcase type frame not more than two jugs high.

Disposable milk jugs develop leaks over time and require routine replacement.  Heavier weight jugs (like returnable plastic milk jugs) last longer.  Other types of containers may be used.  Keep the size two gallons or smaller or water will stratify with hot water on the top and cooler water on the bottom, reducing efficiency.

A passive solar hobby greenhouse is only effective when built to optimum energy specifications.  Because the major heat loss is through the glazing, double-glazing (which reduces heat loss by 25 to 35%) is required.  Double glazed patio door glass is great for glazing a hobby greenhouse.  Glass suppliers sometimes have recycled (used) patio door glass available at minimal prices.  Night curtains may add an additional 30 to 50% energy conservation.  On a passive solar hobby greenhouse, the north, east, and west walls are typically insulated to an R-value of R38.  The foundation and floor are insulated from heat loss to the ground.

Figure 3. Hobby greenhouse being constructed with double glazed patio door glass.

Cold air infiltration is the second major source of heat loss.  For passive solar to be effective, minimize cold air infiltration with good design and construction techniques.  Insulative vent covers help reduce cold air infiltration at night, but must be removed daily to allow thermostats to maintain proper temperature.

A passive solar hobby greenhouse requires an east to west orientation.  In northern Colorado latitudes, an east to west orientation receives 25% more solar energy than a north to south orientation.  Sometimes the hobby greenhouse may be oriented slightly to the east for faster morning warming.  An orientation 20° off east to west will cut 4 to 5% of the solar potential, while an orientation 45° off east to west will cut 18 to 20% of the solar potential.  At northern Colorado latitudes in January, a north to south orientation cuts 25% of the solar potential.

A poorly constructed greenhouse cannot be retrofitted into an efficient passive solar unit.

Cool Season Vegetables

Cool season vegetables do well in the greenhouse or cold frame.  High temperatures are not desirable, and an occasional near freezing dip will not harm crops.  High light intensity is not as critical for cool season crops as for warm season crops. [Figure 4]

General temperatures for cool season crops

• Daytime: 50°F to 70°F
• Daytime short-term temperature extremes: 35°F to 90°F
• Nighttime: 45°F to 55° F
• Germination: 40° to 75°F
• Figure 3. Lettuce in solar greenhouse raised bed

Figure 4. Lettuce in solar greenhouse raised bed.

Table 1. Cool Season Vegetables
Vegetable	Minimum Container Size	Minimum Equal- Distance Spacing	Remarks
Beets	4” deep	6”	Grow in fall and hold in cool greenhouse for winter use Properly thin
Broccoli, Cabbage, Cauliflower	10” deep 5 gallons per plant	18”	High yield for space used. Avoid long-term temperature extremes. Heads split with warm humid conditions.
Carrots	12” deep	3”	Extremely sweet with adequate water and cool temperatures. Use short varieties, like Short & Sweet or Scarlet Nantes. Questionable use of greenhouse space as they can be over-wintered in a garden.
Chard	6” deep	9-12”	Does exceptionally well
Kohlrabi	6” deep	9”	Does exceptionally well
Leaf lettuce	4” deep	9”	Easy to grow in fall, winter and spring in solar greenhouse. Use soft head or leaf types. Keep temperatures under 70°F for sweet quality.
Green onions	6” deep	3”	Never let onions get dry. Sensitive to photoperiod (length of night).  With short days (long nights) cultivars, growth goes into leaf production.  With long days (12-16 hours) cultivars,energy goes into bulb production.
Peas	8” deep	6”	Use dwarf, edible-pod or snap types for salads and stir-fry. Avoid temperature extremes. Questionable use of space due to yields. Don’t transplant well, not well suited to container gardening.
Radish	4” deep	2-3”	Avoid water and heat stress. Must have 12 hours of light to root. Good for fall and spring crops in greenhouse.
Spinach	4” deep	6”	Needs cool greenhouse (45°F to 50°F) for best quality. Avoid temperature fluctuations.
Turnips	4” deep	6”	Good for fall and spring crops
*Deeper container size will make crop easier to care for, providing a larger supply of water and nutrients. Many oriental vegetables are also suited for greenhouse production.

Warm Season Vegetables

Warm season vegetables require high light intensity and moderate night temperatures.  They cannot be cost effectively grown during the winter in a hobby greenhouse without solar heat collectors.  Greenhouse climates control is critical for these fruiting crops to produce.  Warm season crops are not compatible with cool season crops due to differing temperature needs. [Figure 5]

General temperatures for warm season crops

• Daytime – 60°F to 85°F
• Daytime short-term temperature extremes – 50°F to 95°F
• Nighttime – 55°F to 65°F
• Germination – 60°F to 85°F

Figure 5. Beans in solar greenhouse raised bed

Table 2. Warm Season Vegetables
Vegetable	Minimum Container Size*	Minimum Equal- Distance Spacing	Remarks
Beans	8” deep	6”	Not a common greenhouse crop. Good production with adequate light and spacing in spring and fall.  Poor winter production.
Cucumbers	8” deep 2 gallons per plant	18”	Requires high humidity, high light intensity, and good moisture. Needs 75°F to 80° F day temperatures and 50°F minimum nights. Avoid temperature fluctuations greater than 20°F or cucumbers become off flavored. Poor mid-winter production. Plant gynecious greenhouse types. Needs good air circulation to minimize powdery mildew.
Eggplant	8” deep 2 gallons per plant	24”	Hand pollination required.
Muskmelon Cantaloupe	5 gallons per plant	24”	Uses lots of space for yield, try trellising. Needs 80°F day temperatures. Requires hand pollination. Needs good air circulation to minimize powdery mildew.
Peppers	8” deep 2 gallons per plants	15”	Needs night temperatures above 55° F for pollen development. Hand pollination required.
Summer Squash (Zucchini)	5 gallon per plant	24”	Hand pollination required. Needs good air circulation to minimize powdery mildew. Productive with good sunshine.
Tomatoes	12” deep 1-3 gallons per plant	24”	Minimum night temperature of 55°F for pollen development. Hand pollination required. Productive with good sunshine.
*Deeper container size will make crop easier to care for, providing a larger supply of water and nutrients.

Figure 6. Raised bed vegetables in solar greenhouse.

Additional Information

CMG GardensNotes on vegetable gardening

• Block Style Layout in Raised Bed Vegetable Gardens, #713
• Colorado Frost Dates, #740
• Frost Protection and Extending the Growing Season, #722
• Growing Vegetable in a Hobby Greenhouse, #723
• Irrigating the Vegetable Garden, #714
• Mulches for the Vegetable Garden, #715
• Sample Planting Guide for Raised-Bed Garden, #721
• Sample Vegetable Garden Seed Catalogs, #712
• Tomato Early Blight, #718
• Tomatoes – Growing Tomatoes, #717
• Vegetable Garden Hints, #719
• Vegetable Garden: Soil Management and Fertilization, #711     
• Vegetable Gardening in Containers, #724
• Vegetable Planting Guide, #720
• Water Conservation in the Vegetable Garden, #716

Books

• Greenhouses for Homeowners and Gardeners. NRAES.  Cornell University Cooperative Extension. 152 Riley-Robb Hall. Ithaca, NY 14853-5701.  Phone 607-255-7654.  E-mail: NRAES@CORNELL.EDU.  2000. 
• How to Build and Operate Your Greenhouse by Charles Ellwood. HP Books. 1977
• Solar Greenhouses for the Home. Cooperative Extension, Cornell University, Ithaca, NY 14853
• Solar Growing Frame. Rodale Press. 1980
• The Solar Greenhouse Book by James McCullagh. Rodale Press. 1978.

Authors: David Whiting with Carol O'Meara and Carl Wilson: Colorado State University Extension. Artwork by David Whiting.

• CMG GardenNotes are available online at www.cmg.colostate.edu
• Colorado Master Gardener/Colorado Gardener Certificate Training is made possible by a grant from the Colorado Garden Show, Inc.
• Colorado State University, U.S. Department of Agriculture, and Colorado counties cooperating
• Extension programs are available to all without discrimination.
• No endorsements of products mentioned is intended nor is criticism implies of products not mentioned.
• Copyright. 2010-12. Colorado Master Gardener Program, Colorado State University Extension. All Rights Reserved. CMG GardenNotes may be reproduced without change or additions, for nonprofit educational use.

Revised January 2012