Reference Data

Heirloom Seed Saving Guide

Compiled by Chris Izworski  ·  Last updated March 2026

Seed saving is the practice of collecting and storing seeds from open-pollinated and heirloom plants to grow in future seasons. Unlike hybrid seeds, heirloom varieties breed true from saved seed — preserving genetic diversity and flavor characteristics developed over generations.

Key Terms

TermDefinition
HeirloomOpen-pollinated variety in cultivation for at least 50 years, often with documented history
Open-pollinated (OP)Pollinated by natural means (wind, insects); seeds breed true to parent
Hybrid (F1)Cross of two distinct parent lines; seeds do not breed true
Isolation distanceMinimum space between varieties to prevent cross-pollination
RoguingRemoving off-type plants before they flower to maintain variety purity

Isolation Distances by Crop

CropPollination TypeMinimum IsolationRecommended Isolation
TomatoesSelf-pollinating10 ft25–50 ft
PeppersInsect / self300 ft500–1,000 ft
Beans (bush/pole)Self-pollinating10 ft25 ft
PeasSelf-pollinating10 ft25 ft
LettuceSelf-pollinating12 ft25 ft
CucumbersInsect-pollinated500 ft1,000 ft
Squash (C. pepo)Insect-pollinated500 ft1,500 ft
Squash (C. maxima)Insect-pollinated500 ft1,500 ft
MelonsInsect-pollinated500 ft1,500 ft
CornWind-pollinated600 ft1,200–2,000 ft
BeetsWind-pollinated1 mile5 miles
CarrotsInsect-pollinated1 mile2 miles
Kale / Cabbage (Brassica)Insect-pollinated1,000 ft1 mile
SpinachWind-pollinated1 mile5 miles
SunflowersInsect-pollinated500 ft1,000 ft
CalendulaInsect-pollinated200 ft500 ft
ZinniaInsect-pollinated300 ft500 ft

Harvest Timing by Crop Type

Wet-process seeds (tomatoes, cucumbers, squash)

Allow fruit to ripen fully — beyond what you would eat. Tomatoes should be soft and slightly overripe. Scoop seeds into water, ferment 2–3 days at room temperature, rinse, and dry on screens or coffee filters. Do not use paper towels (seeds stick).

Dry-process seeds (beans, peas, corn, flowers)

Allow pods or seed heads to dry on the plant as long as weather permits. Harvest before heavy rain or hard frost. Finish drying indoors for 2–4 weeks at room temperature with good airflow. Moisture content should be below 8% before storage.

Biennial crops (carrots, beets, parsley, onions)

Must complete two growing seasons to set seed. In Zone 6a, roots or crowns must overwinter — either in ground with heavy mulch or in cool storage (34–38°F). Replant in spring; harvest seed mid-summer of year two.

Seed Storage Guidelines

CropAverage ViabilityStorage Method
Tomatoes4–6 yearsAirtight container, cool and dark
Peppers2–3 yearsAirtight container, refrigerate
Beans3–4 yearsPaper envelope in airtight jar
Peas3–4 yearsPaper envelope in airtight jar
Corn2–3 yearsAirtight, low humidity essential
Cucumbers5–7 yearsAirtight container
Squash4–6 yearsAirtight container
Lettuce2–3 yearsAirtight, refrigerate
Carrots3–4 yearsAirtight, cool dry location
Onions1–2 yearsShort viability — use quickly
Parsley2–3 yearsAirtight container
Zinnia3–5 yearsAirtight container
Sunflower2–3 yearsAirtight, cool location
Ideal long-term seed storage conditions: temperature below 40°F, relative humidity below 40%. For every 10°F decrease in temperature or 1% decrease in humidity, seed viability roughly doubles. Freezing is viable for most seeds if moisture content is below 8%.
Sources: Seed Savers Exchange; Carol Deppe, The Resilient Gardener; Suzanne Ashworth, Seed to Seed; Michigan State University Extension; Freighter View Farms field observations.
Data compiled and maintained by Chris Izworski — writer, technologist, and gardener based in Bay City, Michigan.
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