Soaking Seeds: What it Means if Seeds Sink or Float

If you want to produce healthy seedlings, you need to start with viable seeds. Some gardeners claim that testing whether seeds sink or float is a great way to determine if they’ll germinate. But is there any truth to this claim? Join gardener Briana Yablonski as she dives into the world of sinking and floating seeds.

Written by Briana Yablonski Horticulture review by Sarah Jay Last updated: March 12, 2024 | 4 min read

Seeds sink or float test. Pouring chia seeds into glass with water on a wooden table, on a blue background. Some seeds float while others remain at the bottom of the glass. The seeds are small, round in shape, and black in color.

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If you spend time gardening, you’ll hear people spit out all kinds of claims. Some of these turn out true, and others are nothing more than myths passed down through generations and neighborhood email lists. One of these claims relates to seed viability, AKA a seed’s ability to germinate into a seedling.

Many people think you can place seeds in water, look if they float or sink, and use this information to determine whether or not they have the potential to germinate. But is this true? I’ll explain what it means if seeds sink or float and explore other ways you can test seed viability.

The Claim: You Can Test Seed Viability by Seeing if They Sink or Float

Close-up of soaking seeds in water before planting on a seed starter tray filled with soil. The seeds are soaked in a tall glass. The seeds are oval shaped and golden brown in color. — Seed buoyancy doesn’t reliably predict germination, so avoid sink/float tests.

Before I dive into the truth of the matter, let’s clear up the theory in question. Some people claim that placing seeds in water allows you to determine if they will germinate. These people say those that sink will germinate while those that float will not.

However, this isn’t completely true! While some sunken seeds will germinate and some floating seeds will fail to sprout, there isn’t a strong correlation between buoyancy and viability. Generally larger seeds are better for this test than smaller ones. Therefore, you shouldn’t rely on the sink/float test to determine which to discard and which to plant.

A Better Way to Determine Seed Viability

Close-up of a woman's hand holding a package of seeds and reading planting instructions. Nearby there is a stand with many boxes with various packets of seeds of various vegetables, berries and fruits. All seed packets have photos of future crops and instructions. — Check the seed packet for the germination rate, which indicates viability.

If you can’t use the sink/float test to determine seed viability, how can you determine whether or not they will sprout? First, look at your seed packet.

Every reliable seed company will include a germination rate on each seed packet. Companies express this rate as a percentage and use it to express the amount of seeds that will germinate under ideal conditions. For example, if the germination rate is 91%, you can expect 91 out of 100 seeds to germinate.

Each type of seed has a set minimum germination rate; they must meet or surpass this rate if growers wish to sell them. Lettuce and cucumbers have a minimum rate of 80%, while dill and parsley may be as low as 60%. Remember that these rates are minimums, and many seed companies have germination rates well above these numbers.

When you’re looking at a seed packet’s germination rate, take note that it applies to the year they were released. If a seed packet was released in 2024, the germination rate only applies if you plant in 2024. I’ll cover more about the factors that impact germination rates below.

Factors Impacting Seed Viability

While the germination rate tells you how many seeds will sprout during the year they were released, you can’t rely on this rate in the future. That’s because numerous factors decrease this rate as time goes by. Consider the following factors when thinking about germination rate and seed viability.

Age

Various plant and vegetable seeds are soaked in water in different containers on a marble table. Pumpkin seeds are drop-shaped, flat and white in color. Other seeds are tiny, round in shape, and yellow in color. — Seeds have varying viability, with some lasting longer than others.

As seeds mature, their ability to germinate naturally fades. Most flower and vegetable seeds will remain viable for one to five years. Some lose their viability quicker than others, so pay attention to the type of seed when determining whether or not you can expect good germination rates.

For example, onions and parsley only remain viable for one or two years, while cabbage, kale, and tomatoes will keep strong germination rates for four years if stored in a suitable environment.

Storage Conditions

Close-up of three types of seeds spilled from paper bags on a wooden surface. Bean seeds, Chickpeas and pumpkin seeds are on the table. Pumpkin seeds are teardrop-shaped, flattened, and white. The bean seeds are oval in shape, with a glossy smooth pinkish shell and white and black spots that look like eyes. — Proper seed storage in a dry and cold environment ensures viability.

Even if your seeds are only a year old, they may still exhibit poor germination rates if kept in an improper environment. Aim to store your seeds in a dry, cold environment. Some gardeners opt to keep them in a sealed container in their freezer, but a dry cabinet in a climate-controlled house also works well.

No matter your storage spot, make sure they’re away from any sources of moisture. Keeping smaller seed packets in a large airtight bag or container will help protect them from any accidental spills.

Seed Moisture

Close-up of three white paper packets and dry seeds spilling on a stump. These paper bags are labeled with the names of seeds such as Coriander, Sunflower and Dill. — Drier seeds with less than 8% moisture remain viable longer.

Seed moisture is another factor that impacts how long a seed will remain viable. When plants produce seeds, they contain a high amount of moisture. Seed producers then dry them before they sell them since drier seeds remain viable longer than fresh ones.

Seed producers refer to seeds with less than 8% moisture as “hard”. These remain viable much longer than those with more moisture.

Seed Coating

Close-up of a man's hand holding a transparent bag of capsicum seeds on a blurred background. These seeds are flat, round in shape, covered with an inert coating of green color. — Seed coatings improve handling but reduce viability.

Companies sometimes cover small and/or nonuniform seeds with an inert coating to make them easier to handle. For example, uncoated carrot seeds’ irregular shape sometimes clogs the wheels of a push seeder. The coating transforms them into regular, round shapes that travel well through seeders.

While this coating can make them easier to handle, it also decreases seed viability. Therefore, you should only buy the number you plan to use this growing season.

How to Test Germination Rates

Close-up of plastic trays with sprouted pepper seeds on a windowsill. These small seedlings consist of short, upright stems with a couple of cotyledons. These cotyledons are narrow, oblong, oval, dark green. — Test seed viability with a germination rate test.

If you have a packet of outdated seeds or saved seeds from plants you grew, you may be curious about how many will germinate. One way to figure this out is to conduct a germination rate test.

Start by counting out a set number of seeds; if you have hundreds, count out 50, and if you only have 100, count out 10 or 20. Plant these seeds following the instructions on the seedling packet and place them in a suitable environment. When you’re testing the germination rate, it’s essential to provide the ideal temperature, moisture, and light for the seed in question. For example, peppers germinate best at a higher temperature than lettuce.

After they germinate, count the number of seedlings and divide this by the number of seeds you planted. If you planted 20 and only 11 germinated, you have a germination rate of 55%. You can then use this rate to determine how many you need to plant to end up with your desired number of seedlings.

What to Do with Seeds with Poor Germination Rates

Close-up of a small seeding tray with young sprouts. The starting tray is black, plastic, and has square, recessed cells filled with moist soil. Seeds sprouted in almost every cell, but some remained without sprouts. These sprouts consist of thin, pale green stems and three round, smooth, pale green leaves. — Plant extra seeds to account for low germination rates.

If you’re dealing with seeds with low or unknown germination rates, consider planting out more than you think you need. For example, if you hope to end up with ten tomato plants but aren’t sure of the germination rate, plant 20-25 in an open flat rather than individual cell trays. Not all of them will germinate, and that’s okay!

After the viable seeds germinate, you can pot them up into individual cells or containers where they can grow into larger seedlings that are ready for transplanting. This method allows you to end up with the number of mature seedlings you need without wasting soil and space on pots that remain empty.

However, there comes a point when the germination rate is low enough to discredit planting. If less than half germinate, it’s probably time to order some new seeds.

Final Thoughts

It turns out that there’s no point in looking at whether or not seeds sink or float, especially if you’re growing smaller seeds! If you’re concerned about seed viability, examine the germination rate on your seed packet and consider factors like seed age and seed treatment to determine if they are still viable.

Soaking Seeds: What it Means if Seeds Sink or Float

The Claim: You Can Test Seed Viability by Seeing if They Sink or Float

A Better Way to Determine Seed Viability

Factors Impacting Seed Viability