When it comes to the tree of heaven, Lorraine Johnson doesn’t beat around the bush.

“There’s no doubt,” says the author and native plant expert. “It smells like semen.”

The tall, pin-leafed trees of heaven line downtown sidewalks, take over parks and sprout spontaneously in backyards. Their distinct, pungent odour hovers in the air between mid-June and mid-July when the tree flowers.

It’s a smell many recognize, but few are brazen enough to discuss.

Johnson first noticed it about 20 years ago when she lived on Palmerston Sq. near Christie Pits Park. She says there has been a tree of heaven near every place she’s lived in the city. In her last house there was one right outside her bedroom window.

“I moved in right at the stinky season,” she says, laughing.

How’s that for bedroom karma?

The tree’s scientific name is Ailanthus altissima. Its nicknames — sperm tree, semen tree, ghetto palm, stink tree and tree of hell, to name a few — are much more descriptive.

On the West Coast of the United States they’re colloquially known as the Sperm Trees of Los Angeles.

In Lebanon, the trees prompted students at the American University of Beirut to start a group called “I smell sperm every day on my way to Bechtel.”

The tree of heaven was adored when it was first brought to North America from China more than 200 years ago — first as a pretty little garden tree and then as a smart street tree.

These days it’s hardly ever planted. The tree is now considered an invasive species because it re-sprouts vigorously. The tree of heaven can be difficult to get rid of because of how quickly it springs up from seeds and rogue roots.

Where does the scent come from? Well, the first thing you need to know is that the male and female trees are very different.

Both emit a garden-variety yucky smell when their branches are broken or crushed, but only the male produces the flower that sends the tree of heaven’s signature foul odour wafting into Toronto streets.

The males may be guilty for stink, but the female trees spread the seed.

“They produce an awful lot of seed,” says University of Toronto forestry professor Andy Kenney.

The tree can be controlled if its suckers are repeatedly pulled. Hand-digging works but you have to make sure to pull the entire root system and repeat yearly. Pieces of root left in the ground can and likely will re-sprout.

“As long as you were persistent it would eventually die,” Kenney says.

The tree has a few benefits — it provides shade, habitat and is a pleasant-looking tree.

But those benefits tend to be outweighed by its invasiveness and its aggressive expansion into natural areas. Like a weed, the tree of heaven thrives just about anywhere: poor soil, wastelands, alleys, cracks in pavement.

It’s a fertile tree, in more ways than one.

Mark Procunier, manager of tree protection for the City of Toronto, spends a lot of time with the tree of heaven. The smell doesn’t bother him, but he says some Toronto residents call to ask about it.

Loading... Loading... Loading... Loading... Loading... Loading...

“We get a few complaints,” he says. “Not very many. Bitter or pungent are the two terms that I hear.”

Once the blooming is over the smell goes away as quickly as it came.

“It’s just a short window when the trees are in flower,” Procunier says. “It doesn’t last long.”