Complete Cold Frame and Season Extension: From Frost to Harvest
Cold frames and greenhouses extend the growing season by weeks or months. This campaign covers cold frame construction, greenhouse design, heat management, and year-round growing.
Chapter 1: Season Extension Methods
Method
Temperature Gain
Cost
Difficulty
Season Extension
Cold frame
10-20°F
Very low
Very low
4-8 weeks
Hot bed (manure-heated)
20-40°F
Low
Low
6-12 weeks
Low tunnel (row cover)
5-15°F
Very low
Very low
2-6 weeks
High tunnel (hoop house)
15-30°F
Moderate
Moderate
8-16 weeks
Greenhouse (heated)
Full control
High
High
Year-round
Wall o' Water
10-20°F
Low
Very low
4-6 weeks (per plant)
Chapter 2: Cold Frame Construction
Component
Material
Purpose
Specification
Frame (back)
Lumber, straw bale, or brick
Tall side (north)
18-24 inches high
Frame (front)
Lumber, straw bale, or brick
Short side (south)
8-12 inches high
Lid (sash)
Old window, polycarbonate, or plastic
Admit light, trap heat
Hinged at back
Insulation
Straw bales, foam, or earth
Reduce heat loss
Around sides
Hinge
Metal hinge or leather strap
Allow opening
At back edge
Prop stick
Wood stick
Hold lid open for venting
Notched for heights
Cold frame construction: 1) Orient frame facing south (maximum sun exposure). 2) Back wall (north): 18-24 inches high. 3) Front wall (south): 8-12 inches high. 4) Slope from back to front (sheds rain, angles toward sun). 5) Size: match available window sash (common: 3x6 feet). 6) Build frame from 2-inch lumber, straw bales, or concrete block. 7) Hinge sash at back (opens toward north). 8) Seal joints to reduce drafts. 9) Insulate sides with straw bales or banked earth. 10) Prop stick: hold lid open at various heights for ventilation.
Chapter 3: Hot Bed
Hot bed construction: 1) Dig pit 18-24 inches deep inside cold frame footprint. 2) Fill with fresh horse manure mixed with straw (2:1 manure to straw). 3) Pack manure firmly. 4) Manure generates heat as it decomposes (120-160°F initially). 5) Cover manure with 4-6 inches of soil or compost. 6) Wait 3-5 days for temperature to moderate (soil should be 70-80°F). 7) Plant seeds or transplants in soil layer. 8) Hot bed provides bottom heat for 6-8 weeks. 9) After heat is spent, decomposed manure becomes excellent compost. 10) Refresh manure annually for continued heating.
Phase
Temperature
Duration
Action
Initial heating
120-160°F
3-5 days
Do not plant (too hot)
Active heating
70-90°F
4-6 weeks
Ideal growing conditions
Declining heat
60-75°F
2-4 weeks
Still useful, supplement if needed
Spent
Ambient
After 8-12 weeks
Remove and compost
Chapter 4: Hoop House (High Tunnel)
Component
Material
Purpose
Specification
Hoops
PVC pipe, EMT conduit, or bent rebar
Structural arch
Spaced 4 feet apart
Base boards
Treated lumber
Anchor hoops
2x6 or 2x8
Covering
6-mil greenhouse poly
Admit light, trap heat
UV-stabilized
End walls
Plywood, poly, or combination
Close ends
Include door
Ventilation
Roll-up sides or end vents
Temperature control
Essential
Ground anchors
Rebar driven into ground
Secure base
24 inches deep
Hoop house construction: 1) Level site and lay base boards (2x6, 12-20 feet long). 2) Secure base boards with rebar stakes. 3) Bend hoops (PVC or conduit) and insert into ground or base board brackets. 4) Space hoops 4 feet apart. 5) Install ridge pole (connects tops of all hoops). 6) Install purlins (horizontal supports along sides). 7) Pull poly over frame and secure with wiggle wire in channel or batten strips. 8) Build end walls with door. 9) Install roll-up sides for ventilation. 10) Typical size: 12-14 feet wide, 20-96 feet long.
Chapter 5: Temperature Management
Problem
Cause
Solution
Overheating (above 90°F)
Sealed on sunny day
Open vents, prop lid, roll up sides
Freezing (below 32°F)
Cold night, no thermal mass
Add water jugs (thermal mass), insulate
Condensation
Temperature differential
Improve ventilation, angle sash for drip
Wind damage
Unsecured covering
Secure all edges, use wiggle wire
Reference Card
A cold frame is the simplest season extender (an old window over a wooden frame; it adds 4-8 weeks to the growing season at almost no cost). 2. Face south, slope toward the sun (the sash must face south and slope from back to front to maximize solar gain and shed rain). 3. Ventilation prevents cooking (on sunny days, even in winter, a sealed cold frame can exceed 100°F; always vent when temperature rises above 75°F). 4. Horse manure is a heater (fresh horse manure generates 120-160°F as it decomposes; a hot bed provides bottom heat for 6-8 weeks of growing). 5. Water jugs store heat (black-painted water jugs inside the cold frame absorb heat during the day and release it at night, moderating temperature swings). 6. A hoop house is a poor man's greenhouse (PVC hoops and greenhouse poly create a protected growing space at a fraction of the cost of a glass greenhouse). 7. Roll-up sides are essential (without ventilation, a hoop house overheats on any sunny day; roll-up sides provide instant, adjustable airflow). 8. Season extension means food security (growing food 2-4 months longer each year dramatically increases self-sufficiency; cold frames and hoop houses make this possible anywhere).
