City halls and municipalities across the country have reportedly been flooded with calls since media stories about giant hogweed started to spread like… well, a weed.

Indeed, many residents are keeping their eyes peeled for the wicked weed that can grow to five metres tall and emit a sap that can scorch the skin. But it's likely that many reported sightings are false alarms.

Gardening expert Mark Cullen tells CTV's Canada AM that giant hogweed is easily mistaken for other plants in the carrot family, to which giant hogweed belongs.

Queen Anne's lace, for example, can be easily mixed up with giant hogweed. Both have an umbrella-like flower head of white blooms and can grow in all kinds of conditions. Hogweed can also be confused for Angelica, wild carrot, poison hemlock, and others.

But what easily distinguishes giant hogweed are three key features: its giant size, its sharply-cut leaves, and its prickly, purple-spotted stem.

It's the weed's majestic size that likely first attracted Canadian gardeners, who are said to have eagerly bought seeds for the weed at the turn of the last century in hopes of growing the towering wonder in their yards.

But now the weed is taking over roadsides, growing in abandoned lots and along waterways. Cullen says the plant is resilient and can grow in many parts of the country.

"Where it grows is just about anywhere where it's sunny, where it's damp. It likes low land and it's hardy to Zone 3. What's Zone 3? The bottom half of the Canadian prairies. So pretty much most of the country," Cullen said.

If growing the plant is easy, getting rid of it isn't. The plant is hugely invasive and can spread and grow aggressively. Each plant can send out tens of thousands of seeds when it flowers and those seeds can survive on the ground for years.

Rod Krick of the Credit Valley Conservation Authority told CTV News this week that the infestations of the weed that his team deals with are often so large, the only way to eradicate them is with pesticides.

But he says if homeowners find the weed on their land, they can simply cut the flower heads off before it goes to seed. Because it's a biennial, lopping the flower head will stop it from re-germinating and eventually, the plant will die out. What homeowners should not do is to try to pull out the plants themselves; instead, they should call in professional gardening companies that use the right kind of protection.

"It is a very nasty plant and certainly, you do not want to come into contact with it," Krick said.

While some gardeners and homeowners insist the weed can be grown without causing a hazard, Cullen disagrees.

"If you just touched it, the information I have is you won't get a reaction. But if you break the membrane of the hogweed or make contact of any kind with the sap of the hogweed, they you're in for some big trouble," he says.

"Once it's made contact with the skin and the skin has been exposed to sunshine, then it can blister, and it becomes a real mess, much like a poison ivy reaction."

The allergic reaction the plant sap causes is technically called "phytophotodermatitis." While for some it's a mild reaction, for most it leads to burning blisters.

"So some people will react very violently to it; some, not so much. But in any case it's not something to be fooled with," Cullen says.

The trick with giant hogweed is to get the sap off your skin as quickly as possible.

"The antidote is soap and warm water," says Cullen. "So it's just like with poison ivy. What would you do if you made contact with poison ivy? In a short period of time, you should get yourself to a sink with hot water and some soap. That's the best thing that you can do."