TIME FOR A LESSON in winter sowing—sowing seeds in fall and early winter outside in a protected spot, a sort of easy DIY home nursery for making more plants. What we’ll learn to propagate that way are specifically seeds of native plants—both meadow perennials, like asters and Joe Pye weed, and also various shrubs and even trees.
Our guide this time is Heather McCargo, who founded the nonprofit Wild Seed Project in Maine in 2014 and has been growing natives from seed for 35 years. Native plants’ wild populations have shrunk alarmingly in that time. The mission of Heather’s Wild Seed Project is to inspire and teach more of us to grow natives and use them to repopulate the landscape, whether our home gardens or maybe a community project, like at a park or school or beyond. (Wild Seed Project how-to artwork, above, by Jada Fitch.)
Read along as you listen to the Sept. 20, 2021 edition of my public-radio show and podcast using the player below. You can subscribe to all future editions on Apple Podcasts (iTunes) or Spotify or Stitcher (and browse my archive of podcasts here).
winter sowing of natives, with heather mccargo
Margaret Roach: So first, some background: Wild Seed Project, I think it’s a membership organization. It’s nonprofit, and the word “rewilding” comes up a lot on your website, wildseedproject dot net. Explain it and that term to us just briefly.
Heather McCargo: O.K. Well, what most people don’t realize is that all of our developed landscapes are severely depleted in natural processes—from that they’re lacking in the original native plants, and in our planted landscapes most of the plants in gardens now are clones.
So they don’t have the wild traits, and they don’t reproduce because they’re often cultivars, which are selections sometimes chosen because they have particular traits that humans like, like dwarfism or mutated flowers that have bigger or multiple petals. Or they might be double forms, where they have no reproductive organs at all.
And so we’ve lost those wild processes, and rewilding is the movement to restore both nature and the natural wild processes that plants, and all the creatures that are dependent on them, need.
And so the word actually first appeared in the eighties in the conservation movement, and was focused on restoring the large carnivores. Like the case in Yellowstone is very famous because when they brought the wolves back that had been extinct, the wild landscape quickly became much more diverse and healthy, with a lot more life and native plants.
Heather: But at Wild Seed Project, we’re trying to get people to restore this even in our own gardens and backyards. And so having the seed, the genetically diverse seed of our local native plants, is a crucial part to that movement. I’ve been a propagator my whole adult life, and there’s a lot of myths and confusion about the ability to sow, grow plants from seeds. Some of the difficult-to-propagate wildflowers are what cause that, but we have lots of great native plants that are easy to grow from seeds.
Margaret: So we’ll talk about some of those. A lot of people ask me about winter sowing. You know, it’s kind of become a thing. And a lot of plants, winter sow themselves [laughter], kind of—the seed falls in fall or winter onto the ground. I know when we recently did—and thank you for helping me with it—a “New York Times” garden column together about sowing native wild meadow perennials, and so forth, sowing their seed. And you pointed out to me that between mice and birds and who knows what a lot of seed that falls on the ground naturally doesn’t necessarily turn into a plant.
But we can control that a little better with some of our wild-type plants and their seeds and propagate a lot more per plant I think, right? We can get a lot better ratio than if the mice and the birds are eating it by following some of your winter sowing techniques. So what’s the basic idea? Whatever plant we’re working with, what’s the basic setup that I would need to do this, because I don’t just like throw them out in the middle of my backyard or anything.
Heather: Yes. Well, your garden has so many weed seeds. So that’s why I like to get people to sow the plants in pots or flats, or you could make a growing bed. So you need a pot, anywhere from 4 inches to 8 inches across. It can be plastic, it can be clay. You shouldn’t use like a peat pot or one of the biodegradable ones, because these seedlings grow too slowly. They will degrade before. So you need the pot.
You need good organic, compost-based potting soil. And I like the compost in the potting soil because it’s filled with different microorganisms. It’s not sterile. You need a label. And I like to use plastic or some sort of permanent label, and mark it with a pencil, not a pen, most of the magic-marker pens don’t last. And then you need coarse sand to cover the seeds with.
The ideal time to do this is around the holidays, the Christmas-New Year holiday. Not before November—you really need to wait till the cool weather sets in and with the climate change, it keeps getting warmer and warmer in the fall. So you want to wait till all your other outdoor chores are done, and then you can do it inside.
So you fill the pot with potting soil, press it down firmly. You can use the bottom of another pot to press it inside. And then you sow the seeds. And depending on the species depends how much you will cover those seeds.
And this is where the coarse sand comes in. It’s a much better covering for seeds than more potting soil. And the reason is seeds need some light to germinate. When you rototill your garden or dig in your garden, you bring up all these deep seeds from under the soil and that’s why they all germinate. So covering with sand still lets light in and also its coarse, sharp texture helps keep the seeds from splashing out in the rain and prevents damping off. It really is a superior covering.
And a really important thing is to cover each seed the correct depth. A general rule of thumb is to cover the seeds to the depth of the thickness of the seed. So if it was an acorn, you’d bury them an inch. If it was a sunflower seed, you’d do about a quarter of an inch. And if it was a sesame seed, you do an eighth of an inch. And if it’s a teeny dust like seed, you barely cover them at all.
Margaret: O.K. So when we did, as I said, the “New York Times” garden column, we really focused on the meadow perennials. And so just for inspiration so that people, because they have to collect the seed this fall, even though as you said, it’s more as we get toward the holiday period and so forth. And I think at Wild Seed Project, you do kind of a New Year’s sowing, almost a celebratory “looking forward to the future” kind of sowing.
But we would collect the seed as it ripens from perennials. And you mentioned so many in the article, like Penstemon, bee balms, asters, milkweeds. So many others. Maybe you want to mention a few others. Echinacea. Rudbeckia. I have a whole list of them. It was amazing.
[Heather’s recommended meadow perennial list for winter sowing: Penstemon, bee balms (Monarda), asters, and milkweeds (butterfly weed, Asclepias tuberosa, and swamp milkweed, A. incarnata), blue flag iris (Iris versicolor), cardinal flower and blue lobelia (Lobelia cardinalis and L. siphilitica), blue vervain (Verbena hastata), goldenrods (Solidago), ironweed (Vernonia), Echinacea, Rudbeckia, Liatris and Joe Pye weed (Eutrochium purpureum).]
Heather: Yes. And so the woody plants that are still being collected now are the native dogwoods and viburnums. They have a fleshy seed that’s designed be eaten by a bird. You have to clean that flesh off. Pretend you’re a bird. Remove it and then sow them right away. Those seeds don’t like to dry out.
Margaret: O.K. Now with the perennials—I’m going to interrupt for a second. With the perennials, and there were lots of others, like Lobelia and ironweeds and Joe Pye and goldenrods, of course, and cardinal flour. With those, you collect them as they ripen, and I believe you told me I should put them in paper bags properly labeled and let them sit about a month in a cool place to kind of finish ripening before the late-fall sowing.
Heather: Yes. So that’s one thing. These seeds that need to dry out, which is a lot of the fall ones—they need to air-dry for a little while before sowing. In the wild, they’d just be sitting on the stalk of the plant or maybe blowing around. They wouldn’t be down in the wet soil right away.
So yes, that’s important, which is again why you can collect the different species throughout the fall and then hold on to them, and then prepare. While you’re waiting to do your sowing, go get those supplies you need, because it takes more time to go get supplies than it does to do the actual seed-sowing [laughter], say, between the Christmas and New Year holiday.
Margaret: O.K. So the shrubs that you were going to mention, we pretend we’re a bird or a mouse.
Margaret: So you chew off the flesh [laughter].
Heather: Yes. So many of them like the viburnums and dogwoods, they’re an example of those fruits you shouldn’t let dry out. Most of the other ones—bayberry, wild rose, Aronia, the yellow bush honeysuckles (the Diervilla species), buttonbush—those seeds, like with the wild rose that comes in a rose hip, you can actually let those dry and then break them apart with your thumb and get the seed out. But you can sow those seeds dry.
Birches, you can collect those seeds. They disperse off a tree all through the fall and winter. They can be stored dry. Some of the ones that are better off not storing dry are witch hazel, which explode out of their pod usually sometime in October and November. They’re little hard seeds. Those I like to sow right away.
Or then the other common woody plants that people know are the oaks and maples. And those also have seeds that cannot dry out.
Margaret: Now on wildseedproject dot net—just because we can’t cover everything obviously out loud in this short segment. You have great, very comprehensive, detailed explanations of how to do this for each one and which seeds fit into which category, and can and can’t dry out, and so on and so forth. It’s a great reference site. You have a blog, and you have some publications, and lots of good stuff for people who want to dig even deeper into the particulars for one species or another.
Heather: Yes. And so again, especially for beginners to change your thinking and think of cold weather, like I said, the holidays is a great time. That’s the easiest time to sow most of the native species. And you need to be a little more knowledgeable to notice the seed ripening and harvesting. So if that’s over your head, Wild Seed Project sells seeds. And we also have a source on our website of other great native nurseries where you can get native seeds.
But yes, this is a thing, a new way of gardening where you’re restoring the native plants and you’re sowing them in the late fall, early winter.
And you don’t have to worry does that species need one month of cold? Does it need three months or five months of cold? If you sow them outdoors in the late fall, they’ll all get that winter that they need. And then they will germinate starting in the spring.
And some species will germinate as early as March, even when it’s still regularly dipping below freezing. Other species will wait till warmer weather around May and June. So it’s very variable and it’s really interesting and fun to watch. But the important thing for those seeds is that they got to spend their winter outside.
Margaret: Right. So let’s visualize—let’s paint a word picture [laughter] of this little nursery that we’re creating. So we talked about some of the equipment, so to speak. But one of the things that really appealed to me that I saw in the pictures on your site and we talked about for the Times article. It seems to make it more doable and more controlled, and like it’s not going to go astray with some devious animal who’s going to want to disturb all the pots, is to kind of put it inside a frame, almost like a raised-bed frame, or just a simple wooden four pieces of wood kind of thing.
Because we want to cover it with hardware cloth, quarter- or half-inch mesh. We really want to cover it, and weight that down with bricks. Like really, really protect it from the would-be nibblers. Right?
Margaret: So let’s talk a little bit more about it. I mean I feel like a frame would be a great thing, and keep it more organized [laughter].
Heather: Yes. So if you’re handy you can make yourself a wooden frame and then get the little hardware cloth to put on top and you can weight it down with bricks, or you can make a real lid. You can also make a frame with cinder blocks. Let’s say you’re not handy with a hammer and nails, you can just do cinder blocks—make a box and put the hardware cloth above. Doesn’t even hurt to put it underneath.
The important thing about that frame that’s different if you are an experienced vegetable grower, is you want it in the shade for the germinating seedlings.
Margaret: Good point.
Heather: And the reason is come spring, the sun can get hot and strong. And if you leave the house and go to work every day, you don’t want those flats, un-germinated flats, to dry out. Because germination is a process; it’s an event. Once it starts, you can’t stop it. So once that seed starts to germinate, you don’t want it to dry out. That will kill it. And so have those un-germinated pots in the shade.
Now if you were growing something like butterfly milkweed, which is a plant that needs to grow in a sunny, dry site, once those pots have germinated, you want to find a sunny spot to put them in. But for the un-germinated flats, them being in the shade is the best way to get the highest germination rate, because then they won’t have those swings of moisture level and most or all of the seeds will germinate.
And so in those little pots you can sow, for instance, a package from our Wild Seed Project will have anywhere from 40 to 100 seeds in that package. You can sow that whole package in the little pot and cover it with sand.
Native seeds are like teenagers. They like to grow closely together. I could swear they germinate better when you have them all together.
Heather: And then everybody’s really tempted once they do germinate to quickly divide them up, again, especially people who are experienced vegetable growers. These are not annuals. They’re perennials, whether they’re herbaceous perennials or woody shrubs, so they have a slower timetable and they don’t like to be disturbed. So you can take that whole cluster of seedlings and put it in a much bigger pot to grow on through the summer. You know, you can divide them up earlier, but you will disturb the roots so you’ll set their growth back.
Margaret: Right. Sure.
Heather: You see, that’s the advantage of taking the whole clump of seedlings and just putting them in a much bigger pot and keeping them in your little nursery area all summer. It’s not very hard to water a couple pots. And if you have them in a big pot, they won’t dry out all the time, versus planting them out in your garden in June or July.
Unless you’re the most attentive person in the world, you’re going to either lose them to the weeds, or other plants grow in there, because they’re smaller.
Margaret: Yes. Definitely. And people might be thinking like, wait a minute, they’re out in the open all winter long, whatever. But that’s what breaks through the seed coat. That’s what does the job that nature does, right? It gets them to germinate and they each, as you said before, they germinate in their own time, depending on the species and kind of how it’s constructed and its own timeline. It responds to those freezes and thaws and so forth, and then boom, there it goes.
And then maybe what you said, we could transplant the entire clump into a larger pot, grow it on, and then maybe around the next fall, when it’s cooler and moisture again in the garden, would we plant those—maybe divide those up a little more and plant them around the garden? Is that when they’re ready?
Heather: Yes. Now is a great time to do it. And no matter what the winter weather throws at these pots of seedlings, it can be 40 degrees, and then it can dip down to 10 degrees that night, and then get a foot of snow, and then pouring rain. That freeze and thaw, these seeds don’t mind that. They like it.
They actually need it to break up their heavy seed coat. And it’s what’s really different about our native plants. They haven’t been domesticated, which one of the things of domestication is it tends to thin the seed coats. That’s why your lettuce seedlings will all germinate quickly within a week. And if they haven’t, they’re dead. That time of cultivation over the centuries has thinned the seed coats so they germinate really quickly. But wild plants don’t have that, and they need it.
You don’t have to worry about the weather. I’ve had pots of seedlings germinate—I’ve had trumpet honeysuckle germinate in late January in one of those weird winter thaws. And you know what? I just left them and then it got cold many times and snowed and rained, and they were still fine come spring.
And probably in the wild, those seeds germinate kind of under their sort of woodland-edge plant, under the litter. And they’ve learned to germinate in the cool, cold weather of even winter, just the seed, will hang out until it’s time to put on more growth.
Margaret: I want to talk about the setup for our DIY nursery, whatever we’re sowing. We’re going to protect everything, and we’re going to top-dress it with sand, and so on. Do we sow the shrubs and the tree seeds as thickly as you were describing with the meadow seeds, or is there different spacing for those?
Heather: With the shrubs? Yes. I sow them thickly, too. And obviously I do divide, but same thing. You can grow them, the bayberry, wild rose, Aronia you can grow them on as a clump and then wait and divide them in September.
Not the trees…well, depends on the species. I sow, for instance, my birch and maple trees, also maybe a little less close, maybe a half an inch apart. And I also wait to divide them up. They just do better if you let them grow together.
And it’s what often happens in the wild, too. Not all of them then would make it to an adult, but all your pot of seedlings can by separating them out.
A commercial nursery would sooner in the process divide up the seedlings to grow on into the pots. But as a home gardener, leaving them together as a clump, and just keep moving them to bigger pots, they will grow faster because you won’t have the root disturbance—they don’t like that.
Margaret: Yeah. So we should probably in the last minute or two, we should disclaim that we are encouraging people to use wild-type seat as close to the way nature made it as possible. Because of the things you talked about at the beginning, that some of the cultivated varieties have been tinkered with so much that they may even be sterile or not so good at being reproduced this way.
But we’re not saying to run around in wild places and take seed, because that’s normally against the law to go on other people’s property. Right? I mean, it needs to be ethically gathered seed, with permission and so forth, or purchased as you say. Correct?
Heather: Yes. And I consider seed collecting farther down the journey of seed sowing. Start sowing this year, unless you have something right out your back door in your yard, or in a friend’s yard. Just start.
To a collect wild seed you need to properly identify it when it’s in bloom. Most of these plants aren’t in bloom anymore, except for the asters and goldenrod. So that’s a little farther down the journey, but you can get going right away and then you can watch the whole life cycle of the plants.
And the baby seedlings can be quite cute [laughter]. It’s fun to see what they look like. And some look like exactly like just miniature versions of the same leaves, and others have juvenile leaves that are quite different. So it’s a really different way to interact with plants, and participate in a different part of the life cycle that most people don’t get to do anymore, because they just think you buy plants all the time.
Margaret: Right. Right. And I think that’s a really important point to get to recognize the juvenile stage, the seedling stage, of our important native plants. Because, oops, so many times I bet we’ve weeded some of them out, when in fact we could have transplanted them to somewhere where they could mature and thrive, because we didn’t know it was them.
Heather: Yes, absolutely.
Margaret: Yeah. Well, Heather, I’m so excited about your work. And like I said, I’ve already learned so much on the website, wildseedproject dot net. I mean you have, for instance, this publication, “Native Trees for Northeast Landscapes: a Wild Seed Project Guide.” Maybe we’ll talk about that on a subsequent segment we’ll do someday. But lots of, again, really strong resources and inspiration for people who want to learn. And as you say, maybe starting by just buying some seed from you or one of your recommended suppliers and doing it this late fall-early winter, and learn along the way from your website. So thank you so much for making time today. And now get back to your seed collecting [laughter].
Heather: I will. And thank you so much for this opportunity, Margaret.
prefer the podcast version of the show?
MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its 11th year in March 2020. In 2016, the show won three silver medals for excellence from the Garden Writers Association. It’s produced at Robin Hood Radio, the smallest NPR station in the nation. Listen locally in the Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) Mondays at 8:30 AM Eastern, rerun at 8:30 Saturdays. Or play the Sept. 20, 2021 show using the player near the top of this transcript. You can subscribe to all future editions on iTunes/Apple Podcasts or Spotify or Stitcher (and browse my archive of podcasts here).