Seed Formation
Part of Seed Saving
Understanding how seeds develop on the plant is the foundation of all seed saving. Knowing the stages from pollination to mature seed lets you time your harvest perfectly and select the best genetics for future crops.
Every food plant you grow began as a seed, and every seed began as a successful pollination event. To save seeds effectively, you need to understand what happens inside the flower after pollen lands on the stigma, how the embryo develops, and what signals tell you the seed is ready to collect. This knowledge separates gardeners who accidentally get seeds from those who reliably produce high-quality seed stock year after year.
The Pollination Event
Seed formation begins the moment a compatible pollen grain lands on the stigma of a flower. The pollen grain germinates, sending a microscopic tube down through the style toward the ovule. This pollen tube carries two sperm cells. One fertilizes the egg cell to create the embryo. The other fuses with two polar nuclei to create the endosperm, the food reserve that will nourish the seedling during germination.
Double Fertilization
Flowering plants use double fertilization — one sperm makes the embryo, one makes the endosperm. Both must succeed for a viable seed to form. This is why partially filled seed heads are common: some ovules simply did not receive viable pollen.
Pollination Types by Crop
| Crop Type | Pollination Method | Key Detail |
|---|---|---|
| Tomatoes, peppers, beans | Self-pollinating | Pollen transfers within same flower before it opens |
| Corn, spinach, beets | Wind-pollinated | Requires large pollen clouds; plant in blocks, not rows |
| Squash, cucumbers, melons | Insect-pollinated | Depends on bees and other pollinators visiting flowers |
| Wheat, rice, barley | Self-pollinating | Pollination occurs inside closed florets |
Understanding pollination type matters because it determines how easily varieties cross. Self-pollinating crops produce true-to-type seed with minimal effort. Wind- and insect-pollinated crops require isolation to maintain variety purity.
Embryo Development
After fertilization, the single-celled zygote begins dividing. Within days, it organizes into distinct regions that will become the root tip (radicle), the shoot tip (plumule), and the seed leaves (cotyledons). In dicots like beans and tomatoes, two cotyledons form. In monocots like corn and wheat, a single cotyledon develops.
The embryo develops through several recognizable stages:
- Globular stage — A ball of undifferentiated cells forms within the first few days after fertilization.
- Heart stage — The two cotyledon bumps appear, giving the embryo a heart shape (in dicots).
- Torpedo stage — The embryo elongates as the root and shoot axes differentiate.
- Mature embryo — All structures are fully formed and the embryo begins dehydrating for dormancy.
Why This Matters for Seed Savers
Seeds harvested before the embryo reaches maturity will not germinate. The embryo must complete all developmental stages before dehydration begins. Harvesting too early is the single most common seed-saving mistake beginners make.
The entire process from fertilization to mature embryo takes anywhere from two weeks (lettuce) to several months (tree fruits). Annual vegetables typically complete embryo development in 3-8 weeks after pollination.
Endosperm Formation
The endosperm is the seed’s built-in food supply. It develops from the second fertilization event and begins as a liquid that gradually solidifies as the seed matures. Different crops store energy in different forms:
| Storage Type | Examples | Characteristics |
|---|---|---|
| Starchy endosperm | Corn, wheat, rice | Hard, dense, stores well long-term |
| Oily endosperm | Sunflower, flax | Higher calorie but shorter storage life |
| Protein-rich cotyledons | Beans, peas, lentils | Endosperm absorbed into cotyledons during development |
| Fleshy endosperm | Beets, spinach | Moderate storage, seeds remain viable 3-5 years |
In legumes (beans, peas, lentils), the endosperm is consumed during development and its nutrients are transferred into the large, fleshy cotyledons. This is why bean seeds are so much larger than grass seeds — the cotyledons serve double duty as both embryonic leaves and food storage.
Immature Endosperm
Seeds with incompletely developed endosperm will be lightweight and shriveled. They may germinate but produce weak seedlings that struggle to establish. Always select the heaviest, plumpest seeds for your seed stock.
Seed Coat Development
The seed coat (testa) develops from the outer layers of the ovule. It serves three critical functions: physical protection, moisture regulation, and dormancy control. As the seed matures, the coat undergoes significant changes.
Seed Coat Hardening Timeline
Early in development, the seed coat is soft and green. As maturity approaches, several processes occur simultaneously:
- Lignification — Cell walls in the coat deposit lignin, becoming woody and hard
- Color change — Chlorophyll breaks down, revealing species-specific mature colors (brown, black, tan)
- Waterproofing — Waxy cuticle layers develop on the outer surface
- Closure — The hilum (attachment scar) seals, reducing moisture exchange
The hardness of the seed coat varies enormously by species. Bean seeds develop moderately hard coats. Morning glory and canna seeds develop coats so hard they require scarification (scratching or soaking) before they will absorb water and germinate.
Seed Coat Color as Maturity Indicator
For most crops, the seed coat color change is your most reliable visual indicator of maturity. Green seeds are almost never mature. Wait for the species-typical mature color: brown for most beans, black for sunflowers, tan for wheat, dark brown for tomatoes.
Seed Maturation and Drying
The final phase of seed development is maturation drying. The seed systematically dehydrates, dropping from around 80% moisture content to 10-20% moisture on the plant. This process triggers several critical changes:
- Metabolic shutdown — Enzyme activity ceases as water is removed
- Sugar conversion — Soluble sugars convert to insoluble starches and storage proteins
- Heat tolerance increases — Dry seeds can withstand temperatures that would kill hydrated cells
- Dormancy activation — Hormonal changes lock the embryo in suspended animation
Moisture Content at Maturity
| Crop | Moisture at Harvest (%) | Target for Storage (%) |
|---|---|---|
| Corn | 25-35 | 12-13 |
| Wheat | 13-20 | 12 |
| Beans | 15-20 | 10-12 |
| Tomatoes | 45-55 | 6-8 |
| Lettuce | 10-15 | 6-8 |
| Squash | 35-45 | 6-8 |
Seeds that dry on the plant (dry-seeded crops like beans, grains, lettuce) can often be harvested directly at storage moisture levels. Seeds from fleshy fruits (tomatoes, squash, melons) must be extracted and dried separately.
Recognizing Seed Maturity
Different crop families show maturity in different ways. Learning these signs prevents premature harvest.
Dry-Seeded Crops
Grains (wheat, corn, rice): The plant turns golden-brown and the grain resists denting with a fingernail. Corn kernels develop a visible “milk line” that progresses from the top of the kernel toward the tip. When the milk line reaches the tip (black layer formation), the kernel has reached physiological maturity.
Legumes (beans, peas): Pods turn brown, papery, and begin to split. Seeds inside rattle when the pod is shaken. The seeds are hard and resist denting.
Lettuce and greens: Fluffy seed heads (pappus) develop, similar to dandelion. Seeds are dark and easily separate from the head.
Wet-Seeded Crops
Tomatoes: Seeds are mature when the fruit is fully ripe and beginning to soften. Overripe fruit produces the most mature seeds.
Squash and pumpkins: Seeds are mature when the fruit skin is hard and cannot be dented with a fingernail. The stem attached to the fruit turns brown and corky.
Cucumbers: For seed-saving, let cucumbers grow far past eating stage until they turn yellow-orange and soft. Seeds inside will be plump and hard.
Melons: Seeds are mature at eating ripeness. Cantaloupe seeds are ready when the fruit slips from the vine.
Beyond Eating Ripeness
For most seed-saving purposes, seeds need to mature beyond the point where you would normally harvest the fruit for eating. A perfect eating tomato has seeds that are nearly mature. An overripe, slightly soft tomato has seeds that are fully mature. Always let seed-saving fruits ripen as long as possible.
Seed Development Problems
Several conditions can interrupt normal seed formation:
Poor pollination: Insufficient pollen transfer results in partially filled seed heads or hollow seeds. This is common when pollinator populations are low or when wind-pollinated crops are planted in single rows rather than blocks.
Nutrient deficiency: Seeds developing on nutrient-stressed plants will be smaller and lower in stored reserves. Phosphorus is particularly important for seed development. Calcium deficiency can prevent proper seed coat formation.
Drought stress: Water shortage during seed fill causes seeds to mature prematurely at reduced size. The embryo may be viable but endosperm reserves will be inadequate.
Frost damage: A freeze during seed development can kill the embryo outright. Seeds that were close to maturity at the time of frost may still be viable — test with a germination test before relying on them.
Disease: Fungal infections can colonize developing seeds, causing them to rot or introducing seed-borne pathogens that will infect the next generation. This is why fermentation processing (for wet seeds) and proper drying (for dry seeds) are so important.
Selecting the Best Seeds
Understanding seed formation gives you the knowledge to select superior seed stock:
- Choose seeds from the healthiest, most productive plants — you are selecting genetics, not just seeds
- Harvest from the first fruits to set — these had the longest development period and the most mature embryos
- Select the largest, heaviest seeds — these have the most endosperm reserves and produce the most vigorous seedlings
- Avoid seeds from stressed plants — drought, disease, and nutrient stress produce smaller seeds with lower vigor
- Let seeds mature fully on the plant whenever possible — artificial ripening off the plant produces inferior seeds
Key Takeaways
Seed formation is a multi-stage process: pollination triggers double fertilization, creating both the embryo and its food supply (endosperm). The embryo develops through distinct stages over weeks to months. The seed coat hardens and changes color as it matures. Finally, the seed dehydrates and enters dormancy. For seed savers, the critical skill is recognizing maturity indicators — color change, hardness, and drying — and allowing seeds to reach full maturity before harvest. Premature harvest is the most common cause of poor germination in saved seeds. Always select seeds from your healthiest plants, choose the largest specimens, and let them mature as long as conditions allow.