The question of whether viable seeds sink or float has a tricky answer that most gardeners get wrong. Many people believe sinking seeds are alive while floating ones are dead. This test is not reliable for most garden seeds. It only works well for certain large seed types like beans and peas.
I tried the water test for seeds on my tomato collection a few years back and got terrible results. Half of my fresh seeds from packets I had just bought floated to the top of the glass. Those same seeds sprouted with 95% success when I tested them with the paper towel method instead. The float test had me ready to throw away perfectly good seeds.
Small seeds trap air in tiny spaces on their seed coats whether alive or dead. Tomato, pepper, and lettuce seeds float because of their size and shape. The air pockets have nothing to do with whether the seed can still grow. Dead seeds might also contain air from decay, but so do many healthy ones.
Research from Illinois Extension confirms that you can only float test seeds from a few crops with any real accuracy. The test works for peas, beans, and corn because these large seeds sink when full of healthy tissue and float when the inside has dried out or rotted. The weight difference is big enough to matter in water.
When you try to float test seeds from smaller crops, the results mean nothing useful. A tiny tomato seed weighs so little that trapped air bubbles make it float even when fully viable. I watched fresh pepper seeds bob around on the surface right next to old dead ones with no way to tell them apart.
The sink or float seed test can also give you false confidence with large seeds that have problems. A bean seed might sink because of its weight even if the embryo inside died from disease or frost damage. Sinking shows density, not life. The seed coat can look fine while the plant inside is long gone.
Skip the water glass and use the paper towel method if you want real answers about your seeds. Dampen a paper towel, fold 10 to 20 seeds inside, and put it in a plastic bag. Keep it warm for a week or two. Count how many sprout and you have your actual germination rate. This works for every seed size and type.
The paper towel test takes more time than dropping seeds in water. But it gives you numbers you can trust for planning your garden. A week of waiting beats throwing out good seeds or planting dead ones because a float test misled you. Your patience pays off in better germination rates all season.
Save the float test for sorting large quantities of beans or peas before storage. It works fine for removing obvious duds from these big seeds. For everything else in your seed box, the paper towel remains your best friend. It shows you which seeds will grow without any guessing games about floating or sinking.
Read the full article: Seed Viability Test Guide: Ensure Your Seeds Grow
