Stratification Protocols for Selected Tree and Shrub Species sold on Etsy
The information contained here is based on information taken from selected reference books, internet sources and our own growing experience. Our goal is to give you enough information so that you can give your seeds the requirements they need to overcome dormancy and thus germinate. Some species are relatively easy; they require no stratification at all or just a couple of months in moist soil in the fridge. Others take a long time and it may be best to allow these 2 years to germinate. It is these species you don’t see too often in the trade because they aren’t quick to grow and that discourages nurserymen from growing them. Many of these are beautiful plants that deserve a place in our landscapes and chances are some insect or other animal uses the leaves or fruit for food so that makes them valuable to us and to wildlife.
Please note that not all seeds will germinate. Our NAHSeeds are the same ones we use to grow our trees and shrubs and many times the seeds turn out to be duds. There is not a good way to determine whether the seed is viable or not so that’s why we sell them in groups of 20. If you get 25% to germinate then you have 5 seedlings which is great! Othertimes, you’ll get a much higher percentage. It even varies by species and tree to tree. By far the worst offender are Bur Oaks. The big seed is irresistable to weevils and other creatures. They eat the white or pink endosperm in the seed ultimately killing it in many cases. The seeds though have a remarkable “will” to live. We’ve seen partial seeds grow on and become young trees just as well as whole seeds. In preparation to sending them to you we shake the acorn looking for a rattle which means it’s probably bad. And we squeeze the seeds to crack the pointed end to observe the white or pink endosperm. If we see that then the seed is still viable. If we can’t “pop” the seed then that’s a good sign too. Unfortunately, none of these tests necessarily mean the seed will germinate because there might still be a grub feeding in the seed. For our growing purposes in the nursery we try to collect 4X more seed than we think we’ll need to make up for the inevitable loss due to weevils and other factors.
First a couple of definitions:
Stratification: the placing of seeds close together in layers in moist sand or peat to help them germinate.
Scarification: the physical (abrading, sanding, filing away) or chemical (acid treatment to simulate going through a digestive tract) reduction of the seed coat of a seed in order to allow water to penetrate to allow germination.
How We Stratify and Scarify Seeds in the Nursery
Cold, moist stratification for us involves putting the seeds in a moist (not wet) growing medium (like potting mix) and refrigerating the seeds for a prescribed amount of time. Warm moist is very similar but we store the seeds in an office that is room temperature during the day and cooler at night. The potting mix should be moist and not soaking wet. To prepare this mix potting soil and water in a bucket of water and gather a ball of wet media in your hands and squeeze all the excess water out. When your done that media is ready to be used for stratification. We use a 1 gallon ziploc bag in which we place a couple handfuls of moist media and the seeds. We shake it up so that the seeds are in good contact with the media then the bag is labelled with the name of the seed, the source, the current date and the date of completion of the stratification. The bag is refrigerated for the time prescribed time. At the end of that time the seeds are removed from the refrigerator and sown in our greenhouses.
You’ll note that some species below refer to being stored dry. For this we just place the seeds in a ziploc bag without any of the above potting mix. We do not use potting mix on any of our oak species; all are refrigerated dry.
A few of our species need scarifying. Kentucky Coffeetree needs abrasion of the seed coat while Staghorn Sumac need concentrated Sulfuric Acid. Sulfuric acid is very dangerous and I would not recommend you using it. Our Staghorn Sumacs will already have had acid scarification when you buy them so they’ll be ready to grow. We use a belt sander to abrade or nick the seed coat of Kentucky Coffeetrees. We abrade the edge of the seed coat until we seed the white endosperm of the seed. Don’t go too deep. Any bit of white you see is good enough. Now put them in a glass of water overnight and watch how fast they swell up.
No Treatment Necessary
Common Catalpa, Catalpa speciosa – Seeds take a couple week to germinate. A very easy species that grows rapidly. We get 6’+ in one year routinely.
White Oak, Quercus alba – Fall Plant. We’ve had luck sowing them in pots in fall and overwintering them in a garage watering occasionally.
Chinkapin Oak, Quercus muehlenbergii – Fall Plant. Refrigerated seeds rot easily by mid winter. We’ve had luck starting them in pots in fall and overwintering them in a garage watering occasionally.
Species Needing 1 month of Cold, Moist (CM) Stratification
Red Maple, Acer rubrum – Fresh seed planted immediately germinates well. Dry seed could use 1M CM
Silver Maple, Acer saccharinum– Fresh seed planted immediately germinates well. Dry seed rots easily.
Species Needing 2 months of Cold, Moist (CM) Stratification
American Hornbeam, Carpinus caroliniana – Dry seed requires 2M Warm Moist followed by 2 M CM
Scarlet Oak, Quercus coccinea – 2M CM
Pin Oak, Quercus palustris – 1-2M CM
Species Needing 3 months of Cold, Moist (CM) Stratification
Sugar Maple, Acer saccharum – 2-3 M CM
Bitternut Hickory, Carya cordiformis – 3M CM; stored dry in refrigerator
Shellbark Hickory, Carya laciniosa – 3M CM; stored dry in refrigerator. See Aerated Water Soak below
Red Hickory, Carya ovalis – 3M CM; stored dry in refrigerator
Common Hackberry, Celtis occidentalis – 2-3M CM
Swamp White Oak, Quercus bicolor – 1,2,3 M CM; stored dry in refrigerator
Bur Oak, Quercus macrocarpa – 2-3M CM; stored dry in refrigerator
Red Oak, Quercus rubra – 2-3M CM; stored dry in refrigerator. See Aerated Water Soak below
Shumard Oak, Quercus shumardii – 2-3M CM; stored dry in refrigerator
Species Needing 4 months of Cold, Moist (CM) Stratification
Ohio Buckeye, Aesculus glabra – 3-4M CM; Store dry in refrigerator; stored dry in refrigerator
Yellow Buckeye, Aesculus octandra– 3-4M CM; Store dry in refrigerator
Shagbark Hickory, Carya ovata– 3-4M CM; store dry in refrigerator
Silky Dogwood, Cornus ammomum – 3-4M CM
Hazel, Corylus americana – 3M or more of CM
Black Walnut, Juglans nigra – 3-4M CM
Butternut, Juglans cinerea – 3-4M CM
Sycamore, Platanus occidentalis – We place dry seed balls in plastic bags and refrigerate for 2-4 M.
Shingle Oak, Quercus imbricaria – 1-2M to even 4M CM have been reported.
Species Needing Both Warm, Moist and Cold, Moist Stratification
Eastern Wahoo, Euonymous atropurpurea – 2M WM followed by 3M CM. Some come up the following year.
American Hophornbeam, Ostrya virginiana – 3M WM followed by 4M CM. Difficult species.
Bladdernut, Staphylea trifolia – 4-6M WM followed by 3M CM. Our best success was fall planting and having them come up the second spring.
Nannyberry Viburnum, Viburnum dentatum – 5-9M WM followed by 2-4M CM. Alternatively sow outdoors and wait 2 yrs.
Species Needing Physical Abrasion of Their Seed Coat
Kentucky Coffeetree, Gymnocladus dioicus – sand or file through seed coat until you see white endosperm. Soak in water overnight, then plant.
Species Needing Acid Treatment
Staghorn Sumac, Rhus typhina – NAHSeeds are already pretreated and rinsed and are safe to handle. Seeds germinate and grow rapidly.
Species Needing Hot Water Treatment
Black Locust, Robinia pseudoacacia – Fill cup with hot tap water and let stand for 24 hrs. Seeds that have swelled are ready to be planted. Use boiling water on any seeds that have not swelled.
Redbud, Cercis canadensis, Fill cup with boiling water and let cool to 190F. Pour in seeds let soak for 24 hrs. Seeds that have swelled can be place in 3M CM. Any seeds that have not swelled can be retreated.
Sowing the Seeds
The fun part comes when you get to plant them Here’s what we do in our nursery to grow thousands of these every year. We’re presuming that your going to start them in your house, under grow lights, in a greenhouse, etc; not plant them in the ground. Choose a 1 gallon pot and fill with moistened, peat moss based potting mix OR pine fines. Pine fines are a ground pine bark product that is commonly used for landscape mulch. You want it fine. If it’s coarse grade (large chunks of bark) then it won’t work. We use pine fines not garden soil in our pots due to compaction, weed seeds, etc. Lay the seed on its side halfway submerged in the growing media. If these are going to be grown in a sunny, hot area then cover the seed with a thin layer of media to prevent drying out.
If you are directly sowing them in the ground then place the seeds on their sides (not point down) on the growing media halfway buried. If you’re planting this in a sunny or hot location then I would cover with a thin layer of growing media to protect the drying effects of the sun. What you’re planting is squirrel or rodent food. If you have these critters then some sort of protection may be needed. Starting the plants indoors and growing them until they are 1′ or better tall bypasses this problem.
Growing the Seedlings
Once the seeds have sent up their first full leaves and those leaves have expanded you can start fertilizing them. We use a water soluble fertilizer (Miracle Gro-type product) mixed up to label directions. It really doesn’t matter what type of fertilizer you use the plants just need it. If you have a starter fertilizer in your growing media that works too. Every time we water the seedlings get fertilizer. Just make sure you haven’t made it too concentrated; that will kill them.
Aerated Water Soak
The AWS is an interesting way to start seeds. It involves putting the seeds in a half filled, heated and vigorously aerated aquarium for a length of time. The heat stimulates the seed to germinate but the lack of O2 prevents them from growing a root. What it does, in effect, is get them started and to a point where the pericarp (seed coat) splits then the seed stops developing temporarily. This synchronizes all the seeds in the lot and you have a much more uniform germination. You can see which seeds are still alive because they’ve split open and which ones are duds. This saves a lot of time and space in the nursery as we’re certain that what we’re planting is a live seed.
Place the seeds in a half filled aquarium, heat the water with one or two 100W or larger aquarium heaters and aerate the water with two air pumps (or one air pump with two ports) and a submersible power head. We change the water every day or two as tannins and other compounds are leached from the seeds. By Day 4 the Red Oak pericarp has split and begun to open. These seeds can now be planted. Shellbark hickories are the only other species we’ve had luck with using the AWS. What we’ve found is that after about 2 weeks or so the seed cracks open along the central suture line and a thick, white root may start to become visible. Once we see the split we take them out and plant them. Even if it is a slight split we pull them out and plant them.
Doing a Google Search for Aerated Water Soak will provide additional information.
