When the ground freezes in Winter, you may be left wondering what to do with the tree seeds you haven't yet planted.
Luckily, there's a simple way to replicate nature's cold snap for your tree seeds indoors, through cold stratification in your refrigerator!
Cold Stratification is a period of cold which a seed must go through to signal that Winter cold has passed, Spring has come and it's time to wake up and grow.
ABOUT COLD STRATIFICATION
The cold stratification process may seem intimidating, but the basic steps are pretty straightforward, and it's mostly hands off while the seeds hang out the fridge, pretending it's Winter.
Each tree seed variety has specific needs, such as the length of stratification period, need for soaking or scarification, warm and/or cold stratification, etc. You will find this information on each product page under growing instructions. You can also view our master Tree Seed Stratification sheet here.
Here are the basic steps for fridge stratification, a troubleshooting guide to most common issues, and a few tips for successful tree seed storage and germination!
BASIC STEPS FOR FRIDGE STRATIFICATION
- If soaking, scarification, or warm stratification are required, do that first. (see each product page for specific growing instructions)
- Coat the seeds in a damp substrate - peat moss, vermiculite, or garden soil. (There is no need to sterilize your soil; Nature is not sterile and neither are your seeds.) Your substrate should be damp when you squeeze it in your hand, but no water should come out.
- Seal the coated seeds and substrate in a plastic bag or loosely covered container, and put it in the fridge. Check your seed packet or see the product description on the website for information on the number of days required for each variety. Remember to always label your seeds with the name and dates!
- Print out your Tree Seed Stratification Chart, fill it out and stick it on your fridge! Set a reminder on your calendar too and remove the tree seeds when it is time to plant. *It’s very important to leave the seeds in the fridge for the FULL amount of time, or they may not wake up.
- When that time has passed, the seeds will be ready to plant outside or in pots. Young trees should be kept in the shade and the soil should be mulched.
WARM STRATIFICATION FOLLOWED BY COLD STRATIFICATION
Some tree seeds require a warm stratification period followed by a cold stratification period.
This warm stratification period simulates the end of summer when the fruits would be rotting around the seed.
To start, add moist peat or soil to a container or plastic bag with the seeds and give it a shake to coat seeds. Then, put them in a warm place in the house, somewhere it will stay 15-25 degrees Celsius - on top of your fridge works well!
TROUBLESHOOTING TREE SEED PROBLEMS - WHAT TO WATCH OUT FOR!
It is important to keep an eye out for mold during the warm AND cold stratification periods.
It can be normal for a bit of mold to develop, but it should be kept in check. If you do see mold, and it isn’t too bad, you can wash it off with water. If it is severe and blue or green in colour, soak seeds in a 10% bleach solution for five minutes or so, then wash with water.
Then, you can add some new moist substrate and return the *clean* container of coated seeds to its cold or warm stratification place.
Sometimes, there may be some seeds that just don’t seem right. They are squishy when squeezed or seem to be leaking goo…! These seeds need to be removed from the container during stratification. It is important to check for this every month or so.
The main causes of rotting seeds are:
- Too much water in the container. The substrate that gets mixed with the seeds needs to be damp; if it is wet, the seeds are much more likely to rot. If most or all of the seeds are rotting, this is likely the cause.
- The seed had an insect inside or was not viable. If this is the case you will likely only experience rot with a small number of seeds and not the whole container.
THE SEEDS ARE SPROUTING IN THE FRIDGE!
One day, you check on your seeds and find they have all sprouted! This happens more often with some kinds of seeds than others. Ginkgos are one example of tree seeds that have a habit of sprouting early. Sometimes, the seeds may have experienced some of their stratification needs before they arrived in your hands. Depending on how early your tree babies have hatched, you have two choices!
- Leave them in the fridge! The cold will keep growth to a minimum. This can be fine if planting time is a month or so away. Leaving them in the fridge longer may cause some losses.
- Pot them up! If it’s still the middle of winter you’ll have to pot them up. You can have them in the fridge for a month or so, but after that, get out the pots.
TREE GERMINATION RATES AND SEED LONGEVITY - WHAT TO EXPECT
Two things to be aware of with many tree seeds are:
1) Unlike vegetable seeds, tree seeds are short lived and should be started ASAP.
2) Their germination rates are naturally lower and can vary year-to-year.
- Shelf Life. Tree seeds should generally be started as soon as you receive them. They would naturally be sitting on a forest floor covered with damp leaves ready to be frozen by Winter. If you wait months (or until next year...) they will likely not grow.
- Germination Rates. Generally you could expect 30-70% depending on the variety and year grown. This is because of variable weather conditions during the growing season, that can cause a portion of the seeds to not fully develop, even though they may look okay from the outside. Because of the many months of stratification needed for germination and the short shelf life of the seeds, it is not possible to easily test the exact rate of germination, as we do with veggie, herb and flower seeds. From a packet of 10 nut seeds it is possible that you may yield somewhere between 2-6 trees, once the dust settles.
IT’S JANUARY/FEBRUARY - CAN I STILL START TREE SEEDS?
YES! While later than in nature, the timing still works! A good rule of thumb: After adding up all the stratification times, if you are able to finish the process by the end of June, you have time! There will still be time before next Winter for your tree babies to grow strong. Just remember to always mulch your seeds and seedlings when you set them outside in the shade (especially important once summer has started).
Starting trees from seed is a whole new way of interacting with your outdoor world and a whole bunch of fun! We're exited to share our hobby with you and support you along the way. Have fun!