How To Stratify Tree Seeds Indoors

Posted by Hilary & Christopher Mueller on

Revised October 2022

Unlike most vegetable & flower seeds, many Tree Seeds require something called stratification. Nature is amazing & seeds of Northern tree species come equipped with a temperature-based internal clock. It’s stratification that gets this clock ticking!  

Stratification stimulates a seed’s germination process & encourages it to sprout at the right time of year. Some seeds require cold stratification & some require both warm & cold stratification. In nature, these processes happen naturally with the turning of the seasons.

Tree Seeds can of course be planted outdoors, in accordance with Nature’s natural rhythms, but this often doesn’t jive with our modern Human lifestyles. Luckily, there are simple ways to replicate late Summer’s warmth & Winter’s coldness indoors; allowing you to get your Tree Seeds going at any point in the year!

 

STRATIFICATION PREP

The shell of a tree seed is called the seed coat. This is a strong, wood-like layer that protects the seed inside from external threats like decomposition, drought, or being eaten & digested. 🍽️

While a thick, hard seed coat is vital for survival out in the wild, it can hinder uniform germination when tree seeds are being grown by gardeners. Here are some tried & true ways of getting around a hard seed coat. Taking these steps before beginning stratification will greatly improve your results.

 Scarification for Tree Seeds

Scarification 

Some species have a very hard seed coat. Gently scuffing up the surface of these seeds with sandpaper can help water to penetrate & improve germination. Be careful not to go too deep with the sandpaper; you don’t want to damage the inside of the seed. This is only recommended for certain types of seeds.

 

Soaking

For most tree species, soaking the seeds greatly improves germination. Some seeds thrive with a hot water soak, some do well at room temperature & some need to be soaked for 3-4 days. For details, use our Tree Seed Stratification Chart or consult individual packages.

 

 

WARM STRATIFICATION

A few tree species require warm stratification before their cold stratification begins. This warm stratification phase naturally occurs in late-Summer, when fruit ripens, falls & then rots around the seed. This process can easily be simulated indoors.

STEPS FOR INDOOR WARM STRATIFICATION  

(See individual variety webpages or packages for specific growing instructions.)

  1. If a variety requires scarification, do that first.
  2. We recommend soaking most seeds before stratification begins. 
  3. Coat the soaked seeds in a damp substrate - vermiculite, garden soil, etc. Your substrate should be damp when you squeeze it in your fist, but no water should drip out. (There is no need to sterilize your soil; Nature is not sterile & neither are your seeds.
  4. Seal the seeds & substrate in a repurposed plastic bag or food storage container. Label with variety names, as well as the finishing dates of the required warm stratification periods. Remember to always label your containers with names & dates!
  5. Place labeled containers in a warm location in your house; a place where temperatures hover somewhere between 15-25°C. On top of your fridge works well!
  6. Print out our Tree Seed Stratification Chart, fill it out & stick it on your fridge. Set reminders in your phone & on your calendar too! 
  7. Check on the seeds from time to time. You’ll want to watch for mold, as well as the odd squishy seed. Mold is nothing to worry about & is discussed later on. Squishy seeds are not viable & should be removed. The other seeds will be fine.  
  8. Leave the seeds in your chosen “warm place” for the duration of the recommended time. It’s very important for the seeds to experience their FULL warm period or they may not germinate!

When the warm stratification period is complete, you can skip to Step #4 of the below instructions for cold stratification.

 

 

COLD STRATIFICATION

Out in the world, cold stratification happens naturally over the course of the Winter. After a lengthy time spent in cold, damp conditions, the seeds know that Spring has arrived & that it is safe to wake up & grow. You can make it happen right in your own fridge! 

Steps For Cold Stratification In The Fridge 

(See individual variety webpages or packages for specific growing instructions.)

  1. If a variety requires scarification &/or warm stratification, do those first. 
  2. We recommend soaking most seeds before stratification begins. ONLY ONCE
  3. Coat the soaked seeds in a damp substrate - vermiculite, garden soil, etc. Your substrate should feel damp when you squeeze it in your fist, but no water should drip out. (There is no need to sterilize your soil; Nature is not sterile & neither are your seeds.
  4. Seal the seeds & substrate in a repurposed plastic bag or food storage container. Label with the variety names, as well as the dates when the seeds will need to come out of the fridge. Place in the fridge door where temperatures will slightly vary. Remember to always label your containers with names & dates!
  5. Print out our Tree Seed Stratification Chart, fill it out & stick it on your fridge. Set reminders in your phone & on your calendar too! 
  6. Check on the seeds from time to time. You’ll want to watch for mold, as well as the odd squishy seed. Mold is nothing to worry about & is discussed later on. Squishy seeds are not viable & should be removed. The other seeds will be fine.   
  7. Leave the seeds in the fridge for the duration of recommended time. It’s very important to leave the seeds in the fridge for the FULL amount of time, or they may not germinate!

For information on what comes next, here’s our next article: Planting Tree Seeds In Spring. We highly recommend reading this BEFORE Spring arrives… 😘

 

 

COMMON STRATIFICATION QUESTIONS

What Do I Do About Mold?

It is important to keep an eye out for mold during both the warm & cold stratification periods.

Mold thrives in moist environments & it’s entirely possible that you’ll see some while your tree seeds are stratifying. If you do see some, it’s not the end of the world, but mold should be kept in check. So long as it’s not too bad, you can remove the seeds & give them a rinse. Meanwhile, wash & relabel the container. Add some new, moist substrate to the seeds, put them in that clean container & return them to their stratification spot.

If you discover a more severe mold bloom (prolific & blue or green), rinse the seeds & soak them in a 10% bleach solution for five minutes or so. Rinse again with cold water & then repackage them with a new moist substrate, in a newly labeled, clean container. 
 


Some Of The Seeds Are Rotten. What Happend?

Once in a while you may come across a rotten seed. Rotten seeds don’t look right & are often squishy. They should be removed immediately. It is important to check for this every month or so.

The Two Most Common Causes of Rot Are:

  • Too much water in the container. The substrate mixed in with the seeds should be damp, not dripping wet. If conditions are too wet, seeds are more likely to die. If most or all of the seeds are rotting, this is likely the cause. 
  • The seed had insect damage (like a weevil hole) or was simply not viable. This is often the case if just the odd seed is rotten. Remove the rotten seed & have a quick look through the container for the possible insect culprit before returning it to the fridge.

 

Help! There Are Seeds Sprouting In My Fridge!

One day you may check on your seeds & find that they have all sprouted. So exciting! This happens more often with certain kinds of seeds than others. Ginkgos & Hazelnuts both tend to be go-getters & have a habit of sprouting before their cold stratification period is over.
Depending on how early your tree babies have hatched, you have a few choices:  

  • Leave them in the fridge! The cold will hold the seeds in a semi-dormant state & growth will be slow. This is a good option if planting time is about a month or so away. 
  • Pot them up! If Spring is still months away, delicate roots could become too entangled or seeds could run out of space. In this case planting your seeds in indoor pots is the best option.
  • Plant them outside! If Spring has sprung & the soil has thawed, it may be okay to simply plant them out. For details, read our article: Planting Tree Seeds In Spring.

 

How Long Do Tree Seeds Last?

Two things to be aware of with many tree seeds are:

Shelf Life. Tree seeds should generally be started as soon as you receive them. In the wild, ripe tree seeds would 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. When your tree seed order arrives in the mail, get to it asap! 

Germination Rates. Tree seed germination rates are typically lower than common annual crops. Generally, you could expect 30-70% depending on the variety & year of harvest. Variable weather conditions (drought, extreme heat, strong winds) may cause some seeds to not fully develop. Long stratification periods & combined with a short shelf life makes it impossible to test the exact rate of germination, as we do with veggie, herb & flower seeds. From a packet of 10 nut seeds it is prudent to expect a yield of 3-6 trees, once the dust settles.

 

Spring Is Only A Few Months Away - Is It Too Late To Order Tree Seeds?

Nope, you still have time! A good rule of thumb is that if stratification periods can be completed by the end of June, you’ll be okay. There will still be enough time before next Winter for your tree babies to become established. Just remember to always mulch your seeds / seedlings & to keep potted ones in the shade & well watered. This is especially important once Summer has begun.

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Starting trees from seed is another wonderful way of interacting with the world of plants & it’s pretty rewarding. We love sharing this hobby with you & supporting you along the way. Have fun!

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