Washing Seeds

Part of Seed Saving

Washing seeds after wet processing is a straightforward but important step. It removes fermented pulp, gel coats, inhibitory compounds, and debris — all of which can promote mold and disease in storage. More importantly, the washing step includes a float test that distinguishes viable, dense seeds from hollow, infertile, or immature ones. Done correctly, washing produces a clean, high-quality seed lot ready for drying.

Which Seeds Require Washing

Not all seeds need washing. Washing is required for seeds extracted from moist or fleshy fruit through wet processing.

Crop Type	Washing Required?	Method
Tomato	Yes (after fermentation)	Rinse and float test
Cucumber	Yes (after fermentation)	Rinse and float test
Melon, watermelon	Yes (direct extraction)	Rinse and float test
Squash, pumpkin	Yes (direct extraction)	Rinse and float test
Eggplant	Yes (after brief soak)	Rinse and float test
Beans, peas	No	Dry threshing
Corn	No	Dry threshing
Brassicas	No	Dry threshing
Grains	No	Dry threshing
Pepper	Optional (light rinse helpful)	Quick rinse

The Basic Washing Sequence

Regardless of crop, the core washing process follows the same pattern:

1. Place extracted seeds and pulp in a bowl or bucket
2. Add water — roughly 3–5× the volume of the seed mass
3. Work the mass with your hands, breaking up clumps and separating seeds from fibrous material
4. Observe which seeds float and which sink (float test)
5. Skim or pour off floaters and surface debris
6. Pour off the water and remaining suspended pulp carefully, retaining sunk seeds
7. Repeat with clean water 2–4 times until water runs fairly clear
8. Transfer clean, sunk seeds to a drying surface

The Float Test

The float test exploits a simple physics principle: seeds that are fully developed and have dense endosperm or cotyledons are denser than water and sink. Seeds that are hollow, immature, or infertile are less dense than water and float.

Float = discard. Sink = save.

This is one of the most useful quality controls in seed saving because it is fast, free, and requires no instruments. A significant proportion of seeds in any wet-processed lot are infertile or immature — removing them before storage prevents waste of space and container volume.

Salt Water Float Test for Hard-Shelled Seeds

For some seeds where the distinction between viable and non-viable is borderline, a salt water solution (30–50 g of salt per liter of water) is more discriminating than plain water. The increased density of the solution causes more seeds to float, including some that would marginally sink in plain water but are still low-quality. Use 1–2% salt solution for most seeds; rinse thoroughly in plain water afterward.

Float Test Reliability

The float test is highly reliable for:

• Tomatoes (after fermentation removes gel coat)
• Cucumbers (after fermentation)
• Squash, pumpkin (large, clearly dense or hollow seeds)
• Watermelon, melon

Less reliable for:

• Freshly harvested seeds that may have trapped air pockets — tap the surface to dislodge air bubbles before reading
• Seeds from species where mature seeds have naturally low density (some wild species)
• Seeds still coated in pulp — wash first, then float test in clean water

If you are uncertain, err toward saving borderline sinkers and discarding clear floaters.

Water Separation in Detail

Skimming vs. Pouring

Skimming: Remove floating material with a fine sieve, slotted spoon, or your hands. Works well when floaters are sparse. Risk: if you agitate the water, some sinkers may temporarily float.

Decanting: Slowly pour the water at an angle off one side of the container, letting floating debris pour out while seeds remain at the bottom. More thorough than skimming for large amounts of debris. Stop before seeds begin to slide out.

Straining through mesh: After removing large debris by hand, pour the entire contents through a mesh strainer fine enough to retain seeds. Rinse under running water. Works well for large-seeded crops (squash, cucumbers, beans in the rare case of water washing). Not suitable for very small seeds — they pass through most mesh.

Dealing with Stubborn Pulp

Some seeds — particularly watermelon — have pulp that clings tightly and does not rinse off easily. Effective approaches:

• Repeated soaking: Submerge in clean water for 10–15 minutes, then rub gently and rinse. Repeat.
• Light agitation: Rub seeds gently between palms under running water. Avoid aggressive rubbing that damages seed coats.
• Dilute vinegar rinse (optional): A brief rinse in dilute white vinegar (1 part vinegar, 10 parts water) helps break down pectin-based adhesion. Rinse immediately in plain water afterward.

For Fermented Seeds (Tomatoes, Cucumbers)

After fermentation, the gel coat that surrounds seeds has broken down. The seed mass is in a liquid of fermented pulp, dead gel, and microbes. Washing after fermentation:

1. Pour the fermented mass into a larger container and add 4–5× volume of clean water
2. Stir vigorously — dead gel coat and mold will rise; viable seeds will sink
3. Pour off floating material slowly
4. Repeat 3–4 times with clean water each time
5. Final water should be fairly clear; seeds at bottom should be clean and free of gel

Work Quickly After Fermentation

Seeds left too long in fermented liquid after the fermentation period ends begin to deteriorate. Wash seeds promptly once fermentation is complete (typically 2–4 days). Do not let seeds sit in fermenting liquid for more than 5–6 days total.

Washing Tiny Seeds

Very small seeds (parsley, celery, small herbs) present a challenge because they pass through most meshes and are easy to pour off accidentally.

Fine-mesh approach: Use the finest mesh strainer you have — ideally smaller than 0.5 mm opening. If not available, use a tightly woven cloth (thin cotton, muslin) as a strainer.

Slow-pour decanting: Work in a wide, flat container and decant very slowly, watching for seeds to begin moving toward the edge. Stop immediately.

Towel method: Line a strainer with a thin cloth, pour seed/water mixture through, fold cloth and gently squeeze to remove most water, then unroll onto a clean surface to begin drying.

After Washing: Transitioning to Drying

Once seeds are clean:

1. Spread seeds in a single layer on a clean, non-stick surface

   • Glass plates or dishes (best — seeds do not adhere)
   • Ceramic tiles
   • Fiberglass window screen
   • Wax paper or freezer paper
   • Avoid: newspaper (ink may transfer), fine metal mesh (seeds stick)

2. Spread as thinly as possible — ideally one seed deep, not touching

3. Place in a warm, ventilated location away from direct sun

4. Gently stir seeds after 12 hours to prevent them from sticking to the surface and to each other as they dry

5. Continue drying until snap test is passed — typically 7–14 days depending on seed size and ambient conditions

Do Not Use Paper Towels for Initial Drying

Wet seeds stick aggressively to paper towels and are difficult to remove without tearing the seed coat. Use paper towels only after seeds are partially dried (surface no longer wet). Glass and ceramic surfaces allow easy seed removal at any stage of drying.

Rinsing for Disease Reduction

Some pathogens that infect plants are seed-borne — they survive on the seed surface or inside the seed coat. Washing removes surface-borne pathogens, reducing disease carryover to next season’s planting.

Effective against: Bacterial speck, bacterial spot (tomato); angular leaf spot (cucumber); anthracnose (bean, cucumber).

Not effective against: Internally seed-borne viruses; systemic fungal diseases that infect the embryo.

For crops known to carry serious seed-borne diseases, a hot water treatment can be applied before washing:

• Tomato: 50°C for 25 minutes (reduces bacterial canker and other pathogens)
• Brassicas: 50°C for 30 minutes (reduces blackleg, black rot, club root on seed surface)

Use a water bath with a thermometer; temperatures much above 55°C damage germination. Cool in clean water immediately after treatment.

Washing Seeds Summary

Washing removes pulp, fermented residue, gel coats, and surface pathogens from wet-processed seeds. The float test during washing separates dense, viable seeds (sinkers) from hollow, infertile ones (floaters), providing a rapid and reliable quality screen at no cost. Wash with multiple changes of clean water until water is fairly clear, working gently to avoid damaging seed coats. After washing, spread seeds in a single layer on a non-stick surface (glass, ceramic, fiberglass screen) and dry for 7–14 days until the snap test is passed. Do not use paper towels for wet seeds and do not rush drying by using excessive heat.