The concept of “true to seed” (TSS) defines a plant’s ability to produce offspring that are essentially identical to the parent plant when grown from saved seed. This genetic consistency means that if a gardener saves seeds from a specific variety, the resulting plants will reliably display the same traits in appearance, flavor, and growth habits. The predictability inherent in true-to-seed varieties is central to the history of agriculture and remains important for modern seed saving efforts. It allows gardeners to maintain a desired lineage without needing to purchase new seeds every growing season.
The Genetic Foundation of True-to-Seed Reproduction
The biological mechanism that allows a plant to be true to seed is a state of genetic stability known as homozygosity. Within a plant’s cells, every trait, such as flower color or fruit shape, is determined by a pair of genes, with each gene being an allele. A plant is homozygous for a particular trait when both of these paired alleles are identical.
This identical pairing means that when the plant reproduces, it can only pass on one version of that gene to its offspring, ensuring the trait remains unchanged. Plants that achieve this genetic uniformity are often referred to as “true-breeding” lines. Generations of careful selection and self-pollination stabilize these genetics, resulting in a predictable expression of characteristics across all subsequent generations.
Genetic stability provides assurance that the seeds collected will consistently yield plants that mirror the parents, assuming no accidental cross-pollination occurs. If a plant were heterozygous, possessing two different alleles for a trait, its offspring would exhibit variation, causing the plant to no longer breed true. This stable genetic makeup ensures true-to-seed varieties are consistent in yield, size, and flavor year after year.
Open-Pollinated Versus Hybrid Seeds
The ability to breed true to seed is the fundamental distinction between open-pollinated (OP) and hybrid seeds. OP varieties, which include most heirloom types, are inherently true to seed because they are genetically stable and maintained through natural or controlled self-pollination. Provided that pollen is not exchanged with another variety of the same species, seeds saved from an OP plant will grow into a new generation virtually identical to the parent.
Hybrid seeds, designated as F1 (First Filial Generation), result from a deliberate, controlled cross between two distinct, highly inbred parent lines. Plant breeders select these parent lines for specific traits, such as disease resistance or high yield, combining them to produce a superior first-generation plant. The F1 generation is genetically heterozygous, carrying a mix of traits from both parents. This combination often leads to hybrid vigor, resulting in strong, uniform growth.
However, the F1 hybrid itself is not true to seed; its genetic stability ends after that first generation. If a gardener saves and plants seeds from an F1 hybrid, those seeds become the F2 generation, which undergoes genetic segregation. According to Mendelian principles, the mixed traits from the F1 parent recombine randomly in the F2 offspring.
The F2 plants will show a wide range of characteristics, often reverting to less desirable traits from the original grandparents. This results in unpredictable fruit size, uneven maturity, or loss of disease resistance. This genetic breakup means the saved seeds do not produce a plant true to the F1 parent, requiring gardeners to purchase new F1 hybrid seeds each season for consistent results.
Practical Methods for Maintaining Seed Purity
For gardeners preserving open-pollinated varieties, maintaining seed purity requires proactive steps to prevent cross-pollination from other varieties of the same species. Preventing the unwanted mixing of pollen ensures the saved seeds remain genetically true to the parent plant. The required measures depend significantly on how the plant is naturally pollinated, whether by wind, insects, or self-pollination.
The primary method used to prevent genetic contamination is isolation by distance, which involves separating seed-saving plants from potential pollen sources. For self-pollinating crops like tomatoes, beans, and peas, which generally fertilize before the flower opens, a relatively short distance of 10 to 20 feet between varieties is sufficient. Conversely, plants that cross-pollinate via wind (such as corn) or by traveling insects (like squash) require much greater isolation, frequently ranging from a quarter-mile up to two miles in commercial settings.
When adequate distance is not possible, gardeners can employ isolation by timing, which involves staggering the planting dates of different varieties of the same species. Ensuring that the flowering periods of the varieties do not overlap eliminates the risk of cross-pollination. This technique is particularly effective for plants with a short, concentrated blooming cycle.
Physical barriers also help maintain purity, especially for plants relying on insects for pollination. Techniques include caging entire plants with fine netting to exclude pollinators or covering individual flowers with paper bags. Using physical barriers often necessitates hand-pollination, where the gardener manually transfers pollen from the male to the female flower part to ensure successful fruit set.