Frigid temperatures kept me to only one walk this week – and I left with almost no photos when my fingers got too cold to feel the camera’s shutter button! I did see a poor Eastern Bluebird facing south, his fluffed-up feathers blowing in an icy north wind! As I trundled along past brown and bending Queen Anne’s Lace and Canada Goldenrod, filled with powderpuffs of snow, it occurred to me to wonder about how plants and their seeds experience winter. Luckily Stewardship Manager Ben VanderWeide lent me some books about seeds, so I curled up with them and started exploring. Here are the questions I wanted to answer: Where are seeds now? How do seeds get to where they are now? How do seeds know when to “wake up” and germinate?
Where are Seeds Now?
(Click on photos to enlarge. Hover cursor over photo for captions)
By now, most seeds are either on the ground or more likely, in the ground. Dormant seeds waiting in/on the soil are referred to as “soil seed banks.” In fact, according to a wonderfully written book by Jonathan Silvertown, An Orchard Invisible: A Natural History of Seeds, a book I’ll quote throughout this post, “The store of seeds in the soil…may reach tens of thousands per square meter in cultivated ground.” Seeds vary in how long they can stay viable. Some germinate quickly; others can last in the soil for many years, waiting for the right conditions before they germinate. Silvertown mentions a two-thousand-year-old date seed retrieved from the archaeological excavations of King Herod’s palace…,” in other words, in Jesus’ era! And a few years ago scientists were able to germinate 32,000 year old seeds buried with mammoth and woolly rhinoceros bones in Russia. But back to Bear Creek…
Annuals, Biennials, Perennials: Different Survival Strategies
Plants have varying life spans so they have adapted to pass on their DNA in a variety of ways. Wild annuals, short-lived plants, the kind that bother many gardeners as “weeds,” produce abundant seeds that end up in soil seed banks. “Each seed is dispersed in swaddling clothes of maternal tissue.” The “mother plant… can program the layers…to germinate soon or later, and she often goes for a mixture within her brood.” Some will take longer than others to break out of their coatings and germinate, giving the mother plant more chances to produce successful offspring. As Silvertown says, “A seed only ever gets one shot at germination so it must get the timing right.” With a variety of germination times, if the conditions in a particular season aren’t good for germinating, some seeds will stay longer in the seed bank, giving them a chance to germinate in more favorable conditions. Anyone who’s weeded a garden has dealt with some of these annuals that come up out of the seed bank year every year! Some of our annuals are non-native plants like Common Sow-thistle (Sonchus oleraceus) on the left below and some are native plants, like Fireweed (Erechtites hieracifolius) on the right, which waits in the soil until some disturbance exposes the soil surface.
Some of our native wildflowers , like Black-Eyed Susans (Rudbeckia hirta) are biennials. They usually spend their first year producing roots and rosettes (leaves and a short stem) near the ground. Their roots winter over and the second, or sometimes third year, they grow, bloom, are pollinated, make seeds and die.
Most of our native wildflowers, though, are “herbaceous” perennials. For herbaceous perennials the aboveground structures die back in the fall. But underground, their roots, tubers, rhizomes or bulbs survive the winter in the earth and the plant rises from them again the following year. Some, like the Yellow Cone Flowers (Ratibida pinnata) below left also produce seed, carried away by Goldfinches, for example. Woody plants, shrubs and trees like the mighty Oak (Quercus sp.) below right, are perennial plants too, since they live much longer than one or two years. Of course, unlike herbaceous perennials, the aboveground structures of woody plants remain out in the weather year-round. Living cells in the branches and trunk are protected by a sweet “anti-freeze” that trees create in the fall. Dead cells, which make up most of the visible tree, can freeze without harm to the tree.
How Did Seeds Get Where They are Now ?
Plants have adapted a lot of strategies for dispersing their seeds to the relatively safe haven of the ground before winter arrives in our neck of the woods.
Beautiful Wild Columbine (Aquilegia canadensis) drops its shiny black seeds from the follicles that form after the flowers. Such tiny seeds can simply slip into crevices in the earth and be planted by gravity.
Some, like Wild Bergamot (Monarda fistulosa), have two strategies. Along with producing tiny seeds, they also grow in clumps by extending their rhizomes, underground stems that produce new roots and shoots.
Some of the Monarch butterfly’s favorite plants, the Milkweeds (Asclepias species), rely on rhizomes too but they also disperse their seeds in a way that children love, by sailing them on a breeze with an attached parachute called the coma. The advantage of using the fluffy coma is that the plant’s offspring can spread out and not crowd the area where the adult plant grows.
Some seeds have built-in wings, like the Wingstem seeds (Verbesina alternifolia) on the left below. Dr. Ben collected these seeds this year for the Oakland Township seed bank that he keeps stored in a cool, dry place. They’ll be planted in the township parks later this year. You can see the Wingstem plant at this link. Some seeds of trees, called “samaras, ” also come with wings that spin them through the air, slowing their fall and increasing the distance they can travel from the parent tree. The samaras in the center photo are from Box-Elder (Acer negundo), a member of the Maple trees. Some grasses and other larger seeds come equipped with bristles called “awns” that function like feathered darts, sticking up out of crevices after they fall or are driven through the air like arrows. On the right below is a pretty wild grass with awns called Canada Wild Rye (Elymus canadensis) that grows in our township parks. Some awns twist when the humidity changes, drilling the seed into the ground.
Frequently, plants rely on animals to disperse their seeds. The Violet below, which I think is a Common Blue Violet (Viola sororia) – there are 28 Violet species in Michigan! – uses ants to disperse its seeds. Violet seeds have a “fatty wart” attached. Ants haul it into their underground nests for food. Once the “wart” or elaiosome is stripped away, “the ants dump [the seed] …on a trash pile where it can germinate…hidden from predators such as birds.” Big oak seeds, like the Bur Oak acorn (Quercus macrocarpa) below on the right, benefit from the tendency of Gray and Fox squirrels to bury nuts and then forget where they put some of them!
Birds, of course, carry all kinds of seeds, by eating them directly as the Black-Capped Chickadee (Poecile atricapillus)is doing below with the seeds of Queen Anne’s Lace (Daucus carota) or by eating fruits that contain them and depositing the seeds as droppings elsewhere.
Other seeds hitch rides inside burrs that cling to feathers, fur or a passing human, like these seeds from Enchanter’s Nightshade (Ciraea canadensis) that got transported around Bear Creek on the leg of my hiking pants a few years ago.
How Do Seeds Know When to “Wake Up” and Germinate?
Seeds have adapted many ways of figuring out when to germinate. Seeds here in Michigan need to assess temperature, soil moisture and light to germinate. I could imagine how a seed’s coating might soften with moisture and that the warming of spring could trigger growth. But how, I wondered, do seeds “see” light?
Seeds that end up on or near the surface, like Barnyard Grass (Echinochloa muricata) to the left, use light to make their move.
It turns out that molecules on the surface of seeds, called phytochromes, can read the length of daylight, which is often a better indicator of spring than the temperature. Not only that, they can read different wavelengths of light! So they can tell whether the sunlight is falling directly on the seed or whether it’s bouncing off a nearby plant that might compete for sunlight. Depending on what kind of light a seed needs – direct sunlight, partial sunlight or shade – it may start germinating or wait until the right conditions occur. Maybe a nearby tree needs to fall or a larger nearby plant needs to be eaten by an animal before the phytochromes can signal that the light is now right for germination – and then out comes a shoot! Amazing!
Buried seeds, like acorns or violets, obviously can’t use light to determine the seasonal conditions. They are in the dark! But they can use temperature in surprisingly sophisticated ways! If there’s an insulating layer of grass above a plant, the range of temperatures in the soil will not fluctuate much. However, “bare soil has no such insulating layer and so seeds buried beneath it experience extreme fluctuations of temperature.” So seeds of sun-loving plants, for example, will wait to see if conditions improve if the temperature is not fluctuating much. Ones that experience lots of temperature fluctuation know they have bare ground above and may “decide” to germinate. Clever seeds, eh?
The Wonder of Seeds
So like all the life forms we’ve explored together this winter – mammals, birds, reptiles, amphibians and insects – plants too have survival strategies for long Michigan winters. Now, when I watch Milkweed seeds fly by or Wild Senna pods cracking open to release their seeds, I think I’ll have a better appreciation of the magic inside those small packets of life.
Many thanks to Ben for introducing me to the wonders of seeds. I’ll leave you with a thought about seeds that also comes from Silvertown’s book: “Who cannot wonder that the largest organism on this planet, the giant redwood Sequoiadendron giganteum nicknamed ‘General Sherman,’ which weighs roughly the equivalent of a fleet of six Boeing 747Jumbo Jets, germinated more than two thousand years ago from a seed weighing only six-thousandth of a gram!”
Footnote: My sources for information are as follows: Ritland, D. B., & Brower, L. P. (1991); Stokes Nature Guides: A Guide to Bird Behavior Volumes 1-3, Allaboutbirds.org, the website of the Cornell Ornithology Lab at Cornell University; Wikipedia; http://www.butterfliesandmoths.org; Herbarium of the University of Michigan at michiganflora.net; various Michigan Field Guides by Stan Tekiela; Butterflies of Michigan Field Guide by Jaret C. Daniels; University of Wisconsin's Bug Lady at www4.uwm.edu/fieldstation/naturalhistory/bugoftheweek/ for insect info; http://www.migrationresearch.org/mbo/id/rbgr.html for migration info; invaluable wildflower identification from local expert, Maryann Whitman; experienced birder Ruth Glass, bird walk leader at Stoney Creek Metro Park for bird identification; Birds of North America Online; Audubon.org; Nature in Winter by Donald Stokes, Trees in My Forest by Bernd Heinrich, Winter World by Bernd Heinrich, Savannah River Ecology Lab (Univ of Georgia); Tortoise Trust website www.tortoisetrust.org; An Orchard Invisible: A Natural History of Seeds by Jonathan Silvertown,; The Ecology of Plants by Gurevitch, Scheiner and Fox; other sites as cited in the text.