Population Size
Part of Seed Saving
The number of plants you save seed from directly determines the genetic health of your crop in future generations. Too few parents and you trigger inbreeding, bottlenecks, and eventual collapse. Understanding minimum viable population sizes — and why they differ between crops — is essential for sustainable seed saving across multiple seasons.
The Effective Population Size Concept
In genetics, the effective population size (Ne) is the number of individuals actually contributing genes to the next generation. This is often smaller than the census population (total number of plants in a field) because:
- Some plants may fail to set seed
- One plant with exceptional seed production may contribute disproportionately many offspring
- Some plants may be removed (rogued) before they flower
- Unequal sex ratios in dioecious plants reduce Ne
For practical seed saving, effective population size is what matters. Saving seed from 100 plants but only from the ripest 5 means Ne is effectively 5.
Count Contributing Plants, Not Total Plants
When assessing whether you have adequate population size, count only plants that actually contribute seed to your lot — not all plants in the field.
Why Population Size Matters: Allele Retention
Every crop variety carries thousands of alleles (gene variants). Some are common; many are rare. Rare alleles are disproportionately lost when population size drops, because there may be only 1–2 copies in the entire population. If those 1–2 plants are not represented in the seed parents, the allele is permanently gone.
Lost alleles cannot be recreated. They can only be reintroduced from external sources (other seed lots, other varieties). Once gone from a closed population, they are gone forever.
The probability of retaining a rare allele present in only 1 of N diploid plants:
- N = 6 plants: ~71% chance of retaining it
- N = 20 plants: ~90% chance of retaining it
- N = 50 plants: ~96% chance of retaining it
- N = 100 plants: ~98% chance of retaining it
This is why larger populations matter even for “stable” self-pollinating crops.
Minimum Population Sizes by Crop and Purpose
The following table distinguishes between minimum populations for basic seed renewal (maintaining a working seed stock from season to season) and conservation populations (maintaining maximum genetic diversity for long-term adaptation).
| Crop | Breeding System | Minimum (basic) | Recommended (conservation) |
|---|---|---|---|
| Tomato | Self | 6 | 15–25 |
| Pepper | Self (mostly) | 6 | 15–25 |
| Beans (common) | Self | 6 | 15–25 |
| Peas | Self | 6 | 15–25 |
| Lettuce | Self | 6 | 12–20 |
| Wheat, barley | Self | 20 | 50–80 |
| Oats | Self | 20 | 50–80 |
| Rye | Cross (wind) | 50 | 100–150 |
| Corn (maize) | Cross (wind) | 50 | 100–200 |
| Squash | Cross (insect) | 6 | 12–20 |
| Cucumber | Cross (insect) | 6 | 12–20 |
| Carrot | Cross (insect) | 20 | 30–50 |
| Brassicas | Self-incompatible | 20 | 30–50 |
| Beets, chard | Cross (wind) | 20 | 40–60 |
| Onion, leek | Cross (insect) | 20 | 30–50 |
| Sunflower | Cross (insect) | 20 | 30–50 |
| Melon | Cross (insect) | 6 | 12–20 |
Rye and Corn Require Large Populations
Both rye and corn are obligate cross-pollinators that are severely damaged by inbreeding. Never save from fewer than 50 plants of either crop. Below 20 plants, inbreeding depression may become visible within 2–3 generations. Below 10, severe depression (stunted plants, poor seed set, near-sterility) will appear within 3–5 generations.
Consequences of Insufficient Population Size
Immediate (1–2 Generations)
- No visible problems in most self-pollinating crops
- Slightly reduced germination rate possible
- In corn and rye: beginning of inbreeding depression
Short-term (3–5 Generations)
- Reduced plant vigor in cross-pollinating species
- Loss of adaptive capacity — variety responds poorly to unusual weather
- Germination rates may drop to 70–80%
Long-term (6–10+ Generations)
- Severe inbreeding depression in outcrossing species (corn, rye, brassicas)
- Loss of disease resistance traits
- Potential near-sterility in severely bottlenecked corn populations
- Crop may become unreliable under stress conditions
Calculating Minimum Sample Size for Your Planting
A practical formula: if you need to save seed from N plants, you must plant enough to account for:
- Normal germination failure (~20%)
- Roguing of off-types (~10%)
- Plants lost to weather, pests, disease (~15%)
- Plants that fail to produce enough seed (~5%)
Total loss factor: approximately 40–50%.
Required planting = Required seed-parent count ÷ 0.55
Example: You need to save from 20 wheat plants (basic minimum). Plant: 20 ÷ 0.55 = 37 plants minimum. Round up to 50 to provide a comfortable margin.
The Founder Effect and New Populations
When starting a new planting from a small amount of seed — perhaps seed you received from one person, or the remnants of an old collection — you are founding a new population. If the founding seed came from only a few plants, the new population carries only their genetics.
This is the founder effect: the genetic diversity of the founding group permanently constrains the diversity of all future generations.
Practical implications:
- If you receive 20 tomato seeds from a single plant, your entire population is derived from one individual (effectively one founding genotype)
- Crossing with other tomato varieties at the first opportunity increases diversity dramatically
- Never assume that a large seed count means high genetic diversity — origin matters
Strategies for Small-Scale Growers
When space limits the number of plants you can grow, use these strategies:
Rotation saving: Save full-population seed every other year rather than every year. In off years, grow normally without seed saving pressure. This allows full population sizes in seed years.
Cooperative saving: Split seed-saving duties with neighboring growers. One grower maintains one variety; another maintains a different one. Trade seed each generation. Both get diverse seed lots while each only manages one variety.
Emergency consolidation: If a crop is severely reduced by pest or disease, save seed from all survivors regardless of trait quality, then increase population rapidly for several generations before resuming selection.
Bulking: Combine seed from multiple small lots of the same variety (from different sources) into one pooled lot before planting. This temporarily increases effective population size even if planting area is small.
Population Size for Biennial Crops
Biennial crops (carrots, beets, parsnips, many brassicas) complicate population management because they require two growing seasons to produce seed. The seed-parent plants must survive winter, limiting effective population size further.
| Crop | Typical Winter Losses | Adjusted Minimum Planting |
|---|---|---|
| Carrot | 10–30% (varies by climate) | 30–50 roots selected for overwintering |
| Beet | 10–20% | 30–40 roots |
| Parsnip | 5–15% | 25–35 roots |
| Cabbage, kale | 10–40% | 30–50 plants |
In cold climates where overwintering in ground is not possible, roots must be stored indoors (root cellar) and replanted in spring. Storage losses of 20–40% further reduce effective population size and must be factored into initial planting.
Monitoring Population Health Over Generations
Keep records over multiple generations to detect early signs of genetic erosion:
| Metric | How to Measure | Warning Threshold |
|---|---|---|
| Germination rate | Germination test | Below 80% for crops previously above 90% |
| Seedling vigor | Compare to previous year | Noticeably smaller or slower seedlings |
| Days to flowering | Record date | Increasing variability year-over-year |
| Seed set rate | Count plants with poor seed set | More than 10% failing to set well |
| Off-type frequency | Count visibly off-type plants | Rising percentage each generation |
If two or more of these metrics worsen in the same generation, assume genetic erosion is occurring and act: increase population size next season, introduce outside genetics, or bulk up seed from multiple sources.
When in Doubt, Plant More
The cost of planting extra is low. The cost of genetic erosion accumulates silently for years before becoming visible, then is difficult to reverse. When in doubt about whether you have enough plants, plant more.
Population Size Summary
Effective population size — the number of plants actually contributing seed — determines genetic health across generations. Minimum viable populations range from 6 plants for self-pollinating crops (tomatoes, beans) to 50–200 for obligate cross-pollinators (corn, rye). Below minimums, inbreeding depression and allele loss are inevitable over 3–10 generations. Account for germination failure, roguing, and losses when calculating planting quantities. Cooperative saving with neighbors, rotation saving, and periodic bulking can help small-scale growers maintain adequate effective populations.