Season Extension

Season extension is the practice of protecting plants from cold temperatures to grow outside the period defined by your frost dates. The tools range from a single layer of floating row cover to permanent heated greenhouse structures. Most home gardeners operate somewhere in the middle: a few hoop tunnels, some cold frames, and a roll of row cover fabric.

The value proposition is practical. A last frost date of May 10 defines your season start - but with low tunnels and row cover, you can transplant tomatoes three weeks earlier. A first fall frost of October 5 ends your season - but with row cover, your lettuce and kale are still producing in November. You’ve effectively lengthened your productive season by 6-8 weeks at minimal cost.

Row Cover

Floating row cover (spunbonded polypropylene) is the cheapest and most versatile season extension tool. Draped directly over plants or laid over hoops, it traps heat from soil radiation and reduces wind chill.

Standard weights and their protection ratings:

• Lightweight (0.5-0.9 oz/sq yd): 2-4°F of protection; transmits 90% of light
• Medium (1.0-1.5 oz): 4-6°F of protection; transmits 80-85% of light
• Heavy frost blanket (2.0+ oz): 6-10°F of protection; transmits 50-70% of light

Row cover comes in rolls 5-7 feet wide and 25-250 feet long. A 100-foot roll of lightweight cover costs $15-25. It’s reusable for several seasons if removed and stored when not in use.

For tomato transplants, lightweight row cover over wire hoops allows earlier outdoor planting. At night it provides 4°F of buffer; during the day it traps heat and protects against wind. Once daytime temperatures consistently reach 60°F, remove the cover to allow pollinator access.

Cold Frames

A cold frame is an unheated, bottomless box with a transparent lid - typically glass or polycarbonate - that acts as a passive solar collector. Sunlight enters through the lid; the frame traps the resulting warmth. Soil temperature inside a cold frame in zone 6 can remain above freezing through most mild winters, allowing spinach, kale, mache, and overwintering lettuce to produce into December and resume growth in February.

Cold frames are particularly effective in fall because the soil has accumulated heat through summer and releases it slowly. A cold frame placed over a fall planting of spinach can extend harvest by 4-6 weeks past the unprotected frost date.

Low Tunnels and Hoop Houses

Low tunnels use bent rebar, wire hoops, or bent conduit covered with row cover or clear poly to create an enclosed growing environment at ground level. Simple to build with rebar stakes and wire, they protect a 2-3 foot-wide bed and are effective for early spring transplants.

A hoop house is a scaled-up version: bent metal conduit or EMT creating a tunnel 4-8 feet tall and 12-30 feet wide, covered with 6-mil greenhouse poly. A 14x48 foot hoop house costs $800-2,000 in materials, can be built without permits in most jurisdictions, and extends the growing season by 6-8 weeks on each end. In zone 6, a hoop house allows year-round salad production with supplemental heat only during the coldest stretches.

Soil Warming

Black plastic mulch absorbs solar radiation and warms the soil beneath it by 3-8°F compared to bare soil. Used in commercial pepper and melon production, it speeds early growth in cold soils and can accelerate maturity by 1-2 weeks. IRT (infrared transmitting) mulch warms soil more than black plastic while suppressing weeds, but costs more.

Wall-O-Waters and Season Extenders for Tomatoes

Wall-o-Waters are cylindrical structures filled with water that protect individual tomato transplants from frost. Water has high thermal mass; the tubes absorb heat during the day and release it at night, maintaining the interior temperature several degrees above ambient. They allow tomato transplanting 4-6 weeks before the last frost date in many climates. They’re effective but require setup and removal time per plant.