As anyone who has ever purchased seed packs knows, it is a pretty common thing to end up with a shoebox teaming with leftover packs. And it does not take long to accumulate a good selection of seeds from seasons gone by.
One way to make the most of these seeds is germination testing - a process that Sage Garden undertakes each fall and winter to evaluate current-season seeds and to help us plan for seed purchasing ahead of the spring growing season. The main process we use is to set an easy-to-observe quantity of seeds onto a filter paper in a petri dish, add water, close, label and wait. Each day the seeds get checked for germination, re-watered as necessary and the data recorded. At some point (exactly when depends on the variety), the test is deemed complete and the percentage of germinated seeds is compared to reference standards for the various species. For most seeds, germination rates are 85 - 100%, while for a few shy selections, good germination is rated as low as 55%.
Over the years we have noticed a few interesting things about this process. First, there are some seed types that germinate incredibly well in the petri dish but not so much in soil and vice versa. At times, we have found ourselves repeating tests on normally super reliable seeds only to get poor results... then test the same seed-lot in soil and get 100% germination. Some types of squash, cucumbers and beans have behaved this way. Another situation that comes up regularly is seeds that almost immediately go mouldy in a petri dish but germinate easily and without issue in organic potting soil. Examples include nasturtiums, calendula and watermelon. The speed at which some seeds germinate also varies depending on setting; we have seen seeds germinate in less than a day in the petri dish, while in soil the same selection may require 3 - 5 days to emerge.
In some cases, seeds may germinate much more reliably in soil, likely because the soil moisture can be more consistent but this also could be due to a requirement for darkness (some seeds require this - typically larger seeds that you would normally push well into the soil if sowing outdoors).
So, observing seed behaviour through germination testing has piqued our curiosity and changed the way we plan to do certain types of propagation. For one thing, in the greenhouse setting, we are often frustrated by erratically germinating seeds such as ultra-hot peppers and white sage. Traditionally, we started these in soil in 50 cell plug trays but found they would take up a lot of space over a long period, since the seeds may germinate over 4 - 6 weeks. For many of these selections, we moved to germinating them clustered in 4" pots in soil (which by the way seems to improve germination considerably, and a technique we call the Craig Lehoullier method), but there can still be a wide range of germination times and the seedlings end up at rather different stages but needing to be carefully separated before transplanting to plug trays. So, the petri dish method is really helpful to this situation, as we can in a small amount of space watch seeds germinate. And as each seed emerges it can easily be transferred to soil without disturbing the other seeds.
Another amazing thing about using the petri dishes for both germination testing and actually starting seeds is that if you are interested in watching how plants work, you get a very good view! For many seeds, as soon as you add water things start to happen. A fun example includes basil seeds, which develop a purple gel around the seeds within hours of being wetted, demonstrating that they have begun germinating (which is also why basil seeds are used like chia seeds in tropical drinks, similar to the "bubbles" in bubble tea).
The project we have created for the week of January 14 includes:
Two reusable Petri Dishes
Two filter papers to hold moisture round seeds in the Petri dishes
Two 3.5" wood-fibre starter pots
Organic soil for the starter pots
Four kinds of seeds to experiment with (Arugula, Peas, Pink Popcorn & Wheatgrass)
Two wooden plant markers to label seeds started in the pots
It is a little early in the season to start most garden veggies and flowers, but the fun thing here is that you can use this project as an opportunity to learn and observe - and of course, if you want to grow up the seeds that germinate you can do that too! The petri dishes are completely reuseable (just wash with warm soapy water between use), so if you like this as a germination technique you can apply this to future seed starting.
There is no one way to "do" this week's project. Some suggestions would be to experiment with starting the same seeds in both a petri dish and soil, to see what differences occur, or to try the longer-germinators like peppers in the petri dish and see how it is to pluck the germinated seedlings out and plant in a small pot to grow on.
In the petri dish, start by placing the filter paper in the base and drop a little water onto it so that it soaks up; then place seeds in an organized way onto the paper so they stick. Some people place a second layer over the seeds (helps keep them in place), but then you don't get quite the same view! You'll need to carefully rehydrate the filter paper daily (or twice day, if you can - it is quick, don't worry!).
You'll also notice that temperature makes a big difference to germination times... and that in fact some popular seeds like cooler temps (everything brassica, for example) while others really only germinate in a cozy setting (peppers, for sure). This might be another "experiment" to undertake, perhaps by placing one petri dish somewhere warm and another in a cool windowsill, and compare results.
Condensation can form in the lid of the petri dish while the filter paper dries out. this is a sign that the bottom temperature and air temperature are quite different, as is common when using a heat mat. Probably best to remove the bottom heat if this is happening.
One technical challenge we have found with the petri dishes is that if we place them on a heat mat (recommended for many seeds), the water from the filter paper evaporates and then condenses on the underside of the lid; this causes the seeds to dry out quickly hammering germination. So, check for this and be sure to carefully hydrate the filter paper at least once per day. The condensation problem does not normally occur if there is no bottom-heat, but check daily.
We hope this is fun and removes any pressure to do things any particular way or perfectly. This is all about getting up close with seeds and seedlings ahead of the real seed-starting season. Plus, you don't need to have lights, a plant stand or any other particular extra equipment to enjoy this! The side benefits will be watching green things come to life right before your eyes (wait to see how exciting it is to see your first germinator!), and we know that many people won't be able to resist the temptation to grow up the little sprouts.