There is a huge amount of disinformation out there about how to grow fruit tree seeds. Rather than going into all that, this post is about how to do it.
This is not even an experiment. I’ve been growing tree seeds for decades, and I know how it will turn out. This is more a test, to demonstrate the techniques.
I saved a handful of apple seeds, from some random apples I used to make applesauce. Later, I’ll discuss differences in species and varieties, but ordinary apple seeds make a good demonstration.
I divided the seeds into three approximately equal batches, of about 23 seeds.
The first batch was the “control”, or no treatment. I planted these in a pot of dirt, same as you might plant flower seeds.
This was on January 22, 2016.
I kept the pot moist, in normal warm room temperature. At the end of the test, May 4, 2016, there was no difference. Not one had grown.
If you try this with a large number of seeds, you may have one or two random seeds sprout, but this is not the way .
Next, the “freeze” batch, I put overnight in the freezer.
The next day, I put them in a small ziplock bag with damp sawdust, and stored them in the refrigerator.
I checked on them periodically. In this check on April 1, 2016, none have grown.
At the end of the test, May 4, 2016, none had grown.
Looking close, the moisture around the seeds is becoming milky. The seeds are starting to rot. They are dead, and have been since they were frozen.
This is one of the misunderstandings. People hear that seeds need cold to germinate, so they freeze them. As if freezing will “break” dormancy, like shattering ice. No.
If seeds are completely dried, some species can survive deep cold, but it only holds them in suspended animation. It does nothing to make them germinate. Usually, it just kills them, as you see here.
For the “chill” batch, I put them in a ziplock sandwich bag with a little damp sawdust. I put this bag in the refrigerator. These seeds need moisture and oxygen to germinate. Polyethylene, the plastic most ordinary plastic bags are made of, is permeable enough to oxygen that this works fine. You can zip the bag closed so the seeds don’t dry out. You do not need to leave the bag open.
In this check, on April 1, 2016, you can see that some seeds are starting to germinate.
Germination is a gradual thing. I judged that a seed had “germinated” if the root tip coming out was longer than it was wide. So, on this date, I counted 3.
In the next check on April 15, 2016, more had germinated. I replaced the sawdust with a damp paper towel, to make things easier to see. This works just as well as sawdust. If you don’t need the see the seeds, you can use peat moss, sand, soil, crumbled up autumn leaves, etc. If you’re doing a lot of seeds, you can mix them in the bag, and then plant the whole mix once the seeds are germinating.
If you count the seeds, you will notice I did not put back the 3 that had already sprouted. The purpose was to demonstrate germination, not to grow the seeds. If you want to grow the seeds, you can plant them as soon as they sprout. They don’t need cold any more. In fact, cold only slows them down. This is natures way of easing the seedlings into springtime. Chill breaks their dormancy, but then the cold temperatures keep them growing slowly, so they don’t pop up and get hit by late frosts.
Going by my criteria, I did not count the seed in the lower right corner to have “germinated” yet, as its root was not long enough. So I counted 5 more germinated on this date, for a total of 8 so far. But as you will see, it makes little difference in the final tally.
By May 4, 2016, all the rest of the seeds had germinated.
Here’s the numbers on this test. None of the seeds grew except those in the “chill” treatment.
The magic is holding the seeds at a temperature above freezing, but below about 40 degrees Fahrenheit (4 degrees Celsius). Your ordinary home refrigerator is the correct temperature range. How do you know? If it were too cold things would freeze and if it were too warm your food would go bad. If your refrigerator is working normally, it’s perfect to cold-treat seeds. This is what has happened when you cut open an apple and find seeds sprouting inside. The apple was kept in cold storage, and the period of refrigerator temperature satisfied the seeds’ chill requirement.
The seeds need to be imbibed with water, but not under water. Immersed in water, they would not get enough oxygen.
These are like the conditions a seed would find, buried in the surface layer of soil, through the winter. Even in continental climates, where air temperatures go extremely low, the soil a short distance below the surface seldom freezes so cold.
Here are the numbers on the “chill” treatment, with the dates translated into elapsed time. Chill requirements for plants are often expressed in hours, so I’ve included the elapsed time expressed as hours as well as days.
Notice that nothing visible occurred for more than a month! It’s as if the seed has a little hourglass inside, which runs down while the chill conditions are met. If it’s too warm, the hourglass stops. If it freezes, the hourglass stops. But after enough accumulated hours, the seed grows.
Notice that this is not abrupt, but in most batches of seeds, once it starts it goes pretty quick. This is how nature hedges her bets. If some seeds came up too early and got killed, there would be stragglers to take their place. On the other hand, if the early ones got a good head start and took over more space, the next generation would drift towards lower chill. In tree species that have a substantial north-south range, local populations have adjusted themselves to the best chill requirement for their conditions.
Plants use the same chill mechanism to “know” when to leaf out. It correlates with seed chill. That is, for two of the same kind of plant, except with different chill requirements, the one that needs less chill to germinate from seed will need less chill to start leafing out each spring. All else being equal, it will bloom earlier too.
This explains why certain fruits are notorious for getting their blooms frosted. Peaches have a low chill requirement, and apricots even less. Evidently they originated in parts of the world with cold winters, hot summers, and not much transition (chill) in between. Brought to North America, where the weather has all sorts of wild swings, they get their chill requirement at the first breath of spring. They bloom out, then get snapped by late frost.
Why haven’t they adapted? They never needed to. If you grow these seeds, you will find them very “easy” to germinate. Everyone who ever cultivated them did too. The grower planted a bunch of seeds, and the first sprouts to pop up tended to get planted out in the orchard. This, of course, selected for low chill. The trees did well enough, and if the blooms got frosted too often, well, that region would not be known for apricot production.
If you wanted to develop, say, a late-blooming apricot, you could use this approach. Plant a lot of seeds. Say you want ten trees. As the seeds germinate, pot them up, but when the eleventh one germinates, pull up the first one and plant the eleventh one in its place. Keep on doing this. Eventually, you will end up with the slowest to germinate, high-chill individuals. It might take generations, but finally you would have a late-blooming apricot.
This chill technique for germinating seeds works for most mid-latitude tree crops: apples, pears, peaches, apricots, cherries, plums, persimmons, almonds, chestnuts, walnuts, hickories, etc. It may be hard to believe a tender root tip can break its way out of a rock-hard seed like a peach pit or a walnut, but it does.
Most trees that ripen their seeds in the fall need chill. These include ashes, beeches, and some maples such as sugar maple, Norway maple, bigleaf maple, and vine maple. Tree seeds that ripen in the spring tend to have no chill requirement. They are ready to germinate as soon as they fall. These include elms and certain other maples like silver maple, and red maple. A number of leguminous (bean family) trees, such as locusts and mimosa (Albizia), have no chill requirement. Instead, their germination is inhibited by a water-impervious seed coat.
This chill technique is sometimes called “stratification”. This is from a traditional method of putting layers (“strata”) of sand and seeds in flowerpots or lath-bottom boxes, and leaving these sunk in the ground over winter. You can see why it works. Winter moisture keeps the seeds imbibed. The upper layers of soil maintain the proper chill temperatures. The sand makes it easy to separate out the sprouting seeds when it’s time to plant them in nursery rows. If you do it this way, put bricks or something on top of your flowerpots, to keep squirrels from messing with your seeds.
Seeds that need chill will, of course, germinate just fine planted directly outdoors in the fall. However, there is liable to be a lot of grass and weeds also growing by spring when they come up. You may have a hard time spotting the seedlings, or remembering where they were.
If you want to grow a lot of seeds this way, plastic bags may be easier than pots. Collect up your seeds as they come along, and keep them so they do not completely dry out. For example, cherries ripen in early summer. As you use the cherries, put the pits in some loosely lidded container or a plastic bag with something to keep them a little damp, such as moist sand, soil, peat moss, dead leaves, or sawdust.
If you put them in the refrigerator in the summer, they are likely to have their chill requirement met by fall, and germinate as winter is coming on. If this is what you want, growing them indoors all winter, go for it. But for a more natural cycle, put your seeds in a plastic bag and bury it a little below the ground surface, or under some dead leaves or sawdust. Then, the seeds will stay at ambient summer temperatures until autumn, and their timers will not start running down yet.
Be sure you have your seeds buried before things freeze hard. One time, I had a bag of peach pits out on top of the ground, and a November cold snap went to single digits. All these seeds were killed, but the ones buried just a few inches were fine.
As autumn comes, the temperatures will start dipping into the chill range at night, and the seeds’ timers will start ticking. As the weather gets cooler, the seeds will accumulate more chill hours each day. In midwinter, the seeds may go slightly below freezing, and their timers will stop. But as spring comes, they will resume. As each seed germinates, it will start to put out a root. Keep checking them. If left too long, the roots will be all tangled together, and the seeds will be hard to separate out to plant.