Seed Stratification

Stratification is like simulating winter for the seeds, most tree and perennial seeds need to be rehydrated and then kept slightly damp and cold for a couple of months for chemical reactions to occur inside the seed before it will germinate.   The amount of dormancy in the seeds varies between species and also the specific seed lot, so the amount of stratification time required is often an average to get full germination.   Some trees like spruce for example will often germinate without any cold stratification, but with a short period of this cold conditioning you'll get faster and more even germination.

There are indoor and outdoor methods to accomplish this, and often outdoors is easier if you start in the fall.  If you're stratifying the seed before planting it, mix your seeds with some medium and put them outside somewhere sheltered in a container, cover them to keep mice away and store them somewhere sheltered for the winter like beside a shed or garage.  You can also direct seed into beds or pots and let them over winter, we do this into raised beds in our nursery with good results. 

Stratifying indoors is a good option if you start your seeds mid winter.  We usually soak seeds overnight to rehydrate them and start the process, this is easy enough and you can do it in a container or by filling a poly bag half way with water.   Drain the water and for a small amount of seeds add about 1/4 cup of damp medium - which can be anything inert that will hold some moisture like sand, peat moss, coco coir, vermiculite.  Damp paper towel works. For large seeds we use a mix of sand and peat.  If you're gauging how wet to make it, something like 1 tsp of water is enough, you want it to clump a bit and have some even moisture, a minimal amount just to maintain the moisture content in the seeds.  
Sand is handy for small seeds because when you're ready to plant you can rinse them in a strainer and all the sand will wash away - making it easy to find your seeds.   

Label the bag with today's date (sharpie marker, masking tape, or sticker mailing labels work good) and put it in the crisper drawer of the fridge.  The temperature should be cold but above freezing in the range of 1-4C (34-40F).  Don't use the freezer, it can be -18C/-4F and it's too cold and the ice crystallization will usually damage the seeds - outdoors even in a very cold climate the soil temperature under the snow is warmer than your freezer.  Check the bag periodically and make sure the medium is just minimally damp, and as you get closer to the end of the cold conditioning period watch of early germination - some seeds will germinate in the fridge early, which is ok for a short period of time - they just take more care when you plant them.   

If you time stratification to be finished for early spring, you can take the seeds outdoors and spring plant them.   Some seeds take their time to germinate.  We planted some Korean pine this year and they steadily germinated from June through August.   Many will start to grow as soon the soil reaches the right temperature.   Many tree and perennial seeds can handle some frost, if they've germinated naturally in early spring outdoors and some cold weather is coming they'll be fine.    If you decide to germinate them indoors under lights, they'll need to be moved outside carefully after risk of frost has passed and gradually hardened off to the outdoor environment.

This stratification method is just how we do it, and we've germinated many types of trees and perennial seeds this way - but every tree is different, and some germinate better when stratified outdoors.  Some seeds have a deep dormancy, or double dormancy and will only fully germinate after one or more warm/cold temperature cycles - this is a survival adaptation. You can simply it by planting outdoors and being patient, or you can use the fridge method and simulate these cycles by alternate a few months at a time with room temperature, or warm stratification. 

Some seed require scarification, which just means they have a hard or thick seed coat that won't easily let water penetrate.  Sometimes a hot water soak is recommended, which is a process where would add very hot water and then let it cool to room temperature.  Other seeds can be scarified by nicking the seed coat with something sharp, or but rubbing them across sand paper to thin it enough that water can enter.   Acid is sometimes suggested as an option for some very hard seed coats, but our advice is to use a safer method which can be easier, safer and just as effective. 

For sure this a topic you can get more in depth about, and take note if your seeds have any special requirements. You'll find detailed instructions  for stratifying every type of seed with a google search like "(name of plant here) seed stratification". We would encourage you to do your own research and try different methods.  Starting some indoors in the fridge and the rest outside in a pot or a garden bed can be a good way to compare methods.