Is it safe to use native plants from seeds collected far away from where you are planting them?
It might be, depending on what kind of site conditions you are facing and the conditions of the site where the seeds were collected.
[Of course, human-expedited climate change throws a new wrinkle into the debate about provenance.]
But there is one common characteristic used to describe growing conditions that is critical for the wildlife gardener that you probably don’t pay any attention to: chilling hours.
Experienced gardeners know their USDA plant hardiness zone by heart (the part of Baltimore I live in belongs to zone 7a).
The new USDA hardiness zones are extremely accurate, but they basically account for only one factor: minimum winter temperature.
Now, for the record, I choose only locally native plant species grown from seed collected in a local population whenever it is possible.
It isn’t always possible.
For one thing, the seed may simply not be available. Or, if you are working in highly disturbed conditions (like we do in urban parts of Baltimore) there may not BE any nearby wild ecosystems that mimic the soil and microclimate of your planting site.
What happens if I plant a plant grown from seed collected elsewhere?
In addition to being in the same USDA hardiness zone, these cities have generally comparable precipitation levels (46 inches annually for Baltimore versus 53 inches for Marietta).
But despite the same minimum winter temperature, there is a very important difference in the winters Baltimore receives about 1,700 chilling hours while Marietta typically gets only about 900 hours.
Wait, what is a “chilling hour”?
Definitions vary, but a typical calculation involves counting the number of hours each winter that the temperature is below 45 degrees but above 32 degrees.
Okay. But why do chilling hours matter?
Responding to cumulative chilling and subsequent warming is one important way that plants regulate their winter survival and subsequent budding and blooming.
Once the plant has accumulated sufficient chilling hours, the plant readies itself through hormonal changes that control flowering and leaf production once warm weather returns.
And on the flip side, once a plant has accumulated sufficient chilling hours then research suggests that the plant will require a certain number of “warming days” before bud break.
Agricultural growers have long taken chilling hours into account, especially when growing tree fruit. If a plant gets more chilling hours than it needs, the grower is risking frost damage on plants that bud too early. If a plant gets fewer chilling hours than it needs, it may never produce fruit at all.
Now, I don’t grow apples or peaches. But I do grow many native fruit-producing plants for their wildlife value: black cherry, American plum, chokeberry, serviceberry, winterberry holly, viburnum dentatum, several varieties of dogwood, and so on.
These are all larval hosts for local butterflies and moths, but I’m also counting on these plants to produce fruit for adult birds.
If I plant a chokeberry grown from seed collected in Georgia, I run a very distinct risk that it will flower too early and succumb to the late frosts we get here in Maryland.
And conversely, if someone in Georgia plants a chokeberry grown from seed collected here in Maryland they may never get any fruit at all because of insufficient chilling.
Not all plants have a chilling hours requirement (American beech does not, for instance) and for some species the chilling hours requirement may not vary based on the provenance of the plant.
Still, chilling hours can be also be a “hidden factor” in the performance of native plants in your yard. If you have native plants that fail to produce flowers or fruit as you expect, provenance from an area with different climactic conditions may be to blame.
And, remember, cultivars of native plants were selected from stock that was local somewhere: if that “somewhere” is in the same USDA hardiness as you but has dramatically different chilling hours, the plant may reliably survive but never produce satisfactory fruit.
It is therefore important, whenever you are evaluating plants for their wildlife value (whether cultivars or “straight species”) that you evaluate them for yourself or rely on sources that share as many climatic factors with you as you possibly can.
Keep in mind that two places can be in radically different hardiness zones but have similar chilling hours. Brunswick, Maine is in hardiness zone 5b but has roughly the same chilling hours as Baltimore.
Thus, a plant that performs poorly in Pennsylvania may do just fine in South Carolina, or vice versa. Or not.
The conscientious wildlife gardener keeps a studious eye on their garden, and will investigate anomalies vigorously. And now you have one more tool to use.