Two Methods For Testing Seed Viability

For this post in our Ask a Gardener series, Allan N. has asked us about testing seed viability:

Q. I have a bunch of old seeds from years ago. Can I use them in my garden this year? How do I know if they’re still good?

A. Great question! As gardeners across the country are getting ready for their spring planting, and digging out their seeds from last year (or years ago) many are asking the same thing. There are a couple of to test the viability of your seeds before planting.

a collection of opened seed packets stored in a metal tin
Who doesn’t have a stash of old seeds in a box?

Did you know that seeds are actually alive? While seeds are not actively growing, they’re still going through the process of respiration. That means they are slowly converting their stored energy to keep themselves ready for growth as soon as the conditions are right. Seeds uses that energy throughout the germination process, with photosynthesis only taking over after the creation of the plant’s first true leaves.

They can go a long time in this state, but after a few years, they may not have enough energy stored in their endosperm for a good germination and vigorous leaf development. But how long does that take?

Some seeds evolved to stay in this waiting state for a very long time. Many so-called weeds can wait decades (or longer!) to germinate. While other seeds only last a few years at most. Many of the more common garden plants fall into this second group.

vegetable seeds of various types: peas, beans, pumpkin, corn, etc.
Seeds may not look like it, but they’re actually alive

There are a couple of methods for testing seed viability. The two best are the water test, and the germination test.

The Water Test

The quickest way of testing seed viability is to dunk them in a glass of water.

The seeds in question are placed in water for about 15 minutes. After that, you should see some seeds still on the surface and others at the bottom of the glass. The good seeds will sink, while the bad seeds float.

You can then scoop out and throw out the bad seeds, and then confidently plant the good ones. If you’re not planning on planting them right away, just dry them off before storing them again until you are ready.

The Germination Test

This is the preferred method of testing seed viability for most experienced gardeners, as it gives you a good idea of how likely you are to get good seedlings from your old seeds.

cantaloupe seeds on a damp paper towel inside a plastic bag to test if they sprout
Using the germination method for cantaloupe seeds (image source: Downtowngal, Wiki Commons)

It’s pretty simple. Place 10 or more seeds on one side of a slightly damp paper towel, and fold the rest of the towel over the seeds. You can then place the towel in a clear plastic bag and seal it, or put it on a plate and cover it with plastic wrap, depending on how many different types of seeds you’re working with.

Place the wrapped seeds in a warm area (preferably 70 degrees or warmer). On top of the fridge or on a warm windowsill are great spots. Check them every few days to see if any have sprouted. After about 10 days, you should be able to see how many of your original 10 seeds have germinated. This will give you a good estimate of how many of the remaining seeds will sprout as well.

If only 50% of your test seeds sprout, then you’ll know to plant twice as many seeds to make sure you get enough good seedlings for your garden.

Once you’ve figured out which seeds are still good, you’re ready to get started planting! And for those that are past their prime, you’ll know it’s time to buy some new seeds.

Smart Gardener can help

Smart Gardener helps you every step of the way, by creating a personalized garden plan just for you, giving you detailed guidance for planting and tending your garden, and tracking your progress all season long.

Ask a Gardener: finding your frost dates

This is another post in our Ask a Gardener series where you get to ask our resident experts your gardening questions. Send your questions to

Image: frost of leaves and grass; Source: Chandana Ban, Unsplash

Becky D. asks:
I don’t know my frost dates. How can I find that information?

Why it’s important
Gardeners need to know the date of the average last frost in spring and the average first frost in fall because these are used to determine planting dates. The last frost date in spring is a good general guide about when it’s safe to put out plants.

Smart Gardener uses your zip code to automatically determine your frost dates, which works very well if you live in an area with a fairly uniform climate. But if the area covered by your zip code is very diverse (my zip code starts at sea level and goes up to 2500 ft and actually contains more than one climate zone), this may not be an accurate guide for planting, which is why we have included the option to adjust your frost dates in your profile to more accurately reflect your conditions.

In my case, my zip code spring frost date is based on Santa Cruz, which is nearly a month earlier than my actual date. Knowing the frost dates vary dramatically in my area, I looked for more specific information for the area where my garden is located.

Check your local climate center
If you’re in the United States, your best bet is to check the NOAA website for a link to your local Regional Climate Center. I found the Western Regional Climate Center website to be very helpful. My area is covered by the Northern California climate summaries map. This map showed my closest weather station to be in Ben Lomond, so I clicked on that page.

Ben Lomond California Regional Temperature Chart

On the left of the page under Temperatures I found Spring ‘Freeze’ Probabilities. This brings up a graph showing the probability of the occurrence of various temperatures around freezing. Where the orange (32 degree F) line bisects 50% is the average last frost date, which for Ben Lomond I estimated to be about the 23rd of March.

The first fall frost date is also found under Temperatures as Fall ‘Freeze’ Probabilities, and has a similar orange (32 degree) line. Where this bisects 50% is the average first fall frost date. I estimate this to be about the 12th of November for Ben Lomond.

There is also a ‘Freeze Free” Probabilities page, which has a graph showing the length of your frost free season. For Ben Lomond I estimate this to be about 240 days.

Smart Gardener Frost Date Settings

Editing your garden planner
Once you know your average last frost date, you can enter that manually in your garden’s settings in the Smart Gardener app. Next to your garden’s name at the top, select “Settings.” Under “Growing Season Settings” you can edit your first and last frost dates based on what you learned from your Regional Climate Center’s information.

Smart Gardener will use these updated dates to determine your best dates for planting the vegetables you selected for your garden, guiding you on when to start your seeds and when to transplant your seedlings outside.

Things can change
It’s sometimes said, “Climate is what you expect, and weather is what you get.” Keep in mind that these frost dates are averages and so of course they will vary from year to year (some years it will be earlier and some years it will be later).

But following the info above will give you a good idea of how to determine your garden’s last spring frost date and first fall frost date, and that will be a good place to start.