Just as our own experiences shape our response to the future, trees remember their lives as seedlings and it shapes their responses to environmental stress.

Arborists had long suspected a “nursery effect” in which transplanted trees of the same species seemed to respond differently to the same environment depending on the nursery where they were grown. A 2011 study by the University of Toronto at Scarborough used poplar tree nursery stock to examine this theory.

Poplar trees (Populus sp) are propaganted clonally so a cutting grown from a parent tree is genetically identical to the parent. The study obtained stem cuttings from the same parent poplar tree regrown in widely separated nurseries in Alberta and Saskatchewan. They then regrew the trees in Toronto under identical conditions with half exposed to drought, the other half well watered.

Amazingly the clones from Alberta responded differently than those from Saskatchewan. They even used different genes in their response.

“The findings were really quite stunning,” said Malcolm Campbell, lead author of the study. “Our results show that there is a form of molecular ‘memory’ in trees where a tree’s previous personal experience influences how it responds to the environment.”

That’s why it’s unwise to transplant a tree grown in Somerset County, PA to a backyard in Pittsburgh. The origin and destination climates are too different. The tree’s triggers are incorrectly set for its new life. (Somerset is zone 5b, Pittsburgh is 6b, on the Plant Hardiness Map).

This applies to forest trees too, even though they aren’t transplanted. Their previous experience could help their survival in the face of climate change, diseases and pests.

As winter arrives this week, watch the trees respond with their own history as a guide.

Resources: Lab Girl by Hope Jahren and Forest trees remember their roots from Science Daily

(photos by Kate St. John, map from USDA)

It’s tree-planting season and a good time to remember that trees can be damaged by our good intentions. In the old days we staked every newly planted tree but we’ve since learned that for most tree plantings, stakes are a bad idea.

Tree trunks become strong from the ground up by swaying in the wind. When a tree is staked, it “thinks” it already has strong roots where it’s staked so it puts effort into growing tall instead of establishing roots. The trunk becomes strong above the yoke and remains weak below it. In addition the yoke may damage the trunk, further weakening the tree as shown above.

If your new tree has a big root ball it probably doesn’t need to be staked, though there are exceptions quoted here from the Davey Tree blog. You should use stakes on …

• Bare-root trees or trees with a small root ball.
• Trees planted in areas with lots of foot traffic, like a sidewalk or street.
• New trees that can’t stand on their own or those that begin to lean.
• Eucalyptus trees, mesquite hybrid trees, oleander trees and acacia trees.
• Tall, top-heavy trees with no lower branches.
• Young trees if you live in a very windy area or if the soil is too wet or loose.”

If you use stakes make sure to remove them at the next growing season. If you don’t, the tree will grow around them like this one did at Schenley Park. See more photos at How Stakes Hurt Trees.

Still have questions? Get expert advice at Davey Tree blog, Gardening Know-how, or Northern Woodlands.

(photo credits: Wikimedia Commons (click caption for the original), and Kate St. John)

Even when a species is invasive, nurseries sell it and people plant it until it’s banned. Consider the case of Leylandii trees, sold as shrubs in the U.K.

At the garden center, the homeowners say, “I want something that grows quickly and provides some privacy.” They bring home these cute little shrubs.

But Leylandii grow three feet a year. Eventually the shrubs are so tall they have to be trimmed using ladders, like the “ultimate green square” in the photo at top.

The hedges eat the bus stop …

… and engulf the lane.

On Throw Back Thursday read how these “privacy” shrubs can spark fights among the neighbors in this vintage post: Plant A Shrub, Start A Fight.

(photos from Wikimedia Commons; click on the captions to see the originals)

p.s. Shakespeare fans will recognize my allusion to Hamlet’s madness.

Now that black locusts (Robinia pseudoacacia) are leafing out in Pittsburgh we can watch their leaves do exercises.

According to Wikipedia black locust leaves fold together at night and during wet weather, a trait of the Legume family called nyctinasty. I’ve often seen nyctinasty in clovers but have never noticed it in black locusts because I haven’t been paying attention. This month I plan to take a look.

It will be a good week to get close to black locusts. They’re blooming now with a sweet grape-like scent. See photos and read more about them in last year’s article: The Sweet Smell of Trees.

Yesterday I noticed white flowering trees on the hills and swales near Robinson Town Centre (I-376 West). “How nice,” I thought, “Who planted those trees in the empty places?” No one. They’re invasive.

April is the perfect time to see the invasive extent of callery pears (Pyrus calleryana) because they bloom before our native white-flowering trees: chokecherry, downy serviceberry (shadbush), and hawthorn.

Originally imported from China in the early 1900s as root stock for pear orchards, USDA bred them as landscape trees in the 1950s and came up with a winning cultivar, the thorn-less sterile “Bradford pear.”

From 1960 to the 1990s callery pears were wildly popular as street trees in suburbia. They’re pretty in early spring, colorful in fall, and they grow well in the full sun and disturbed soil found in new subdivisions. The Bradford cultivar is also brittle so commercial plant breeders created other cultivars. That’s when the genie came out of the bottle.

In a single cultivar population the fruits are sterile but if two different cultivars are planted near each other, or even grafted together, insects cross-pollinate them and the trees produce fertile fruit. Birds eat the fruit and disperse the seeds. The trees escape to the wild.

Callery pears grow anywhere. A patch can start with a single tree that becomes a thicket in several years. Dense thickets push out all native species. To make matters worse, the wild trees can have 3-inch thorns! The best field-scale control measure is to brush-hog and then mow every year. They still come up!

Callery pears now grow wild from Texas to New York and Massachusetts. They’re listed as invasive in eight states including our own: Pennsylvania, Maryland, Virginia, South Carolina, Georgia, Alabama, Tennessee, and Illinois.

You’ll see them this month on open hillsides and fields along the interstate, near shopping centers, and at the edges of subdivisions.

Don’t plant callery (Bradford) pears. They are not nice flowering trees.

(photos by Richard Gardner and Britt Slattery via bugwood.org; click on the captions to see the originals)