John Manion is a guy who know his plants. That's not surprising since he's the curator of the seven-acre Kaul Wildflower Garden, the native plant collection at Birmingham Botanical Garden in Birmingham, Alabama.

What may surprise you is that he's come down with his fair share of rashes and itching from poison ivy. The problem isn't that he doesn't know what poison ivy looks like. He can readily recognize it as well as poison oak and poison sumac, the three most common poisonous plants gardeners, homeowners and folks who just like to take a walk in the woods are likely to encounter. And poisonous in this case means the plants will cause a blistering, oozing rash that itches so badly it's difficult to resist the temptation to claw your skin off to stop the pain.

The problem for Manion is that poison ivy is so prevalent that it's almost impossible for him to avoid it during the countless hours he spends in the field, as well developing, documenting, researching and interpreting the gardens' collection. "I feel like it comes with the territory, and I'm not too concerned about getting it," he said, adding that "I'd rather get it than chigger bites or ticks."

He gets it that few people have to deal with poison ivy, poison oak and poison sumac as an occupational hazard and even fewer are willing to suffer the consequences of them no matter where or how they might encounter them. He also understands that if he knows what these plants look like and still comes in contact with them, members of the public are really susceptible to accidentally encountering them. To help people avoid the misery they cause or a trip to the doctor's office as a last resort for seeking relief, he offered some tips on how to identify each of these plants.

But, first, he offered some interesting facts about them.

One is that there's only one thing the active ingredient in poison ivy, poison oak and poison sumac can bond with: human skin. That ingredient is urushiol, an oily mixture of organic compounds with allergenic properties. "It can get on your tools, your clothing, your shoes or the fur of a pet, but soap and water will take it off easily," said Manion, adding that "a lot of people don't realize that." Be aware, though, he said, that unless you wash those items, the urushiol will linger on them and can transfer from them to your skin. You can also wash it off your skin if you wash the affected area almost immediately with soap and cool water, scrubbing hard with a washcloth. That can be difficult, especially if you didn't even know you touched one of these plants in the first place. "Unless you do that, it will penetrate the epidermis within minutes," Manion said. Once that happens, no amount of washing will stop the inevitable rash and itching.

Another fact about poison ivy, poison oak and poison sumac is that coming in contact with them can be more risky in winter than in summer. The winter risk is because the plants are deciduous, meaning they will drop their leaves, which are one of the primary ways to identify them.

A third is that all three are primarily Eastern plants. "There is a different species of poison ivy in the western United States, so that's why poison ivy is sometimes called Eastern poison ivy," Manion pointed out. It has a wide distribution and can range all the way up into Canada and into Newfoundland.

Here's an ID guide that will provide some further clues to keep your time outdoors enjoyable and itch-free.

Poison ivy (Toxicodendron radicans)

poison_ivy As poison ivy ages, the leaves turn from light to dark green. (Photo: Rob Byron/Shutterstock)

"Poison ivy is the most common of the three by far," said Manion. "It grows in a variety of habitats and is basically everywhere. What makes it different from poison oak and poison sumac is that it can take different growth forms. It can be a small shrub, it can creep along the ground almost like a ground cover and it can climb into surrounding shrubs or up into a tree. I saw it on Long Island once, and no one would even believe when they first looked at it that it was poison ivy. The vine attached to the tree was about three inches in diameter. That was the biggest example of poison ivy I have ever seen."

Here's how to recognize it in each of its various forms.

First look at the leaves. You've no doubt heard the saying, "leaves of three, let it be," or some variation of that. The saying is generally true for all growth habits of poison ivy, but it isn't botanically accurate for any of them, said Manion. "Poison ivy doesn't have three leaves, although that's what most people call them." Instead, it has leaves that consist of three leaflets. Look carefully and you'll notice that the leaf has two side leaflets directly connected to a central stem and a third leaflet, the terminal one, the one on the end, on a little stem-like extension.

There are some other less-well-known features about the leaves that will help you identify poison ivy. One of those occurs in the spring. When the plants are first leafing out, Manion said the leaves will have a brownish-red tint to the tips of the new foliage. As the leaves mature, he said, they will almost always turn a deep green rather than a light or pale green. Frequently, the leaves will also have a little bit of a shine, although that isn't always the case.

One feature of the leaves that Manion said isn't a reliable identification feature is the shape of the edges. In some cases, the edges are jagged (toothed, in botanical terms) and in others they are smooth.

There is only one other plant Manion said he is familiar with that people sometimes mistake for poison ivy. That is boxelder maple (Acer negundo). At first glance, boxelder maple looks just like poison ivy because it has three leaflets. But, Manion said, there's an easy way to tell the difference. Look carefully at how the leaflets are placed along the stems. On boxelder maple, the leaflets are exactly opposite from each other. whereas on poison ivy they alternate, or are staggered along the stem. "That's a really, really good way to tell the difference."

There's also a telltale way to identify poison ivy when it's growing as a vine. In this form, the vine can resemble a hairy rope and makes very hairy roots that help it cling to the bark of the tree. Botanists call these adventitious roots, which simply means roots growing where you wouldn't normally expect to see roots growing — in this case out of the stem of the vine as it clambers upward on the tree. "Oftentimes, you'll see that once the vine is attached to the tree, the plant's branches will actually stick out in a horizontal pattern four to five feet," said Manion. As with poison ivy growing as a shrub or ground cover, poison ivy vines will also have three leaflets.

The ropey look, Manion said, has led to another saying about how the casual observer can identify poison ivy when it grows as a vine: "Bark like a rope, don't be a dope." The rope-like appearance of the stem, though, isn't a reliable way to ID a poison ivy vine in winter.

Our native climbing hydrangea, sometimes called wood vamp (Decumaria barbara) is another common native plant that grows as a vine that also has a stem with a rope-like appearance. Climbing hydrangeas are easy to identify from spring to fall by either their rounded leaves or cream-colored flowers, which appear in small clusters. Determining the difference between it and a poison ivy vine in winter is an entirely different matter, even for an expert like Manion.

"You have to be very careful in the winter when there are no leaves to be able to differentiate between them," Manion said. "If the two were next to each other in the winter without foliage and I didn't have anything else to go on, I wouldn't touch anything that has that ropey bark."

Yet another way to help the casual gardener, homeowner or hiker identify poison ivy are the clusters of berries the plant produces. At first, they are green, but as they mature they turn white with a waxy-like coating. The berries are about the size of those on beautyberry (Americana callicarpa), though the shrubby beautyberry looks nothing like any form of poison ivy. Poison ivy berries are an important source of food songbirds, which aren't bothered by the urushiol, and help the plant spread through undigested seeds in their droppings.

Another caution about poison ivy is in the beautiful hues the leaves can take on in the fall. Colors can range from shades of red to yellow to orange. If you are out in the woods collecting leaves for arrangements, don't make the same mistake Manion said some Europeans reportedly made many years ago. "I heard a story once about some Europeans who were so enthralled with poison ivy's scarlet fall color that they took it back to Europe as an ornamental."

As with many plants, there are anecdotal stories about poison ivy that may or may not be true. One regarding poison ivy is that when it grows as a vine its leaves can sometimes mimic those of the host plant. "I never heard that one," Manion said.

Poison oak (Toxicodendron pubescens)

Toxicodendron pubescens Poison oak only grows a couple feet in height and isn't a vine like poison ivy. (Photo: Sundry Photography/Shutterstock)

Poison oak is nowhere near as common as poison ivy. "I spend a lot of time in the field, and in all of my countless hours I've seen it about three of four times," Manion said.

Poison oak also occurs in leaflets of three, but what makes it hard to distinguish from poison ivy is that its leaflets look just like those on poison ivy. Other times, the leaflets will resemble a white oak leaf, a shape from which the plant gets its common name.

There are several growth habits that can help distinguish between poison ivy and poison oak. One is that when Manion said he has seen poison oak, it has always been in drier conditions than where he has seen poison ivy. The other thing is that he said poison oak, to his knowledge, doesn't climb. "The tallest it gets is one or three feet. You are never going to see it climbing up a tree as a vine. The only real distinct ID feature I can give about poison oak because it can look so similar to poison ivy is sometimes you see that oak leaf shape."

The point, of course, regardless of the leaf shape, remains. "Leaves of three, let it be" unless the leaflets are opposite from each other on the stem. If the leaflets are staggered, regardless of whether they are the telltale oak shape, the painful itch will be the same if you come into contact with it.

Poison sumac (Toxicodendron vernix)

Toxicodendron vernix Out of all three poisonous plants, poison sumac grows the tallest; but you're unlikely to encounter it. (Photo: Andy Jones, Cleveland Museum of Natural History/Flickr)

The last in the trio of poisonous plants doesn't look like either of the first two.

Poison sumac can grow into a large shrub or small tree that can get as tall as eight or 10 feet and produces numerous leaflets, with each leaf having as many as 10 or more leaflets. It has the furthest westward range of the three and can grow as far west as Texas.

Manion recalled a clue he has always considered as a sure-fire way to help him identify poison sumac. "A friend of mine told me that if where you are standing is not wet, you are not seeing poison sumac. You are never going to see poison sumac where it is dry. It grows on the edges of bogs, seeps or swamps." In addition, the central stem of poison sumac that holds all of the leaflets is frequently reddish.

This is one to definitely avoid, Manion advised. "I have read that of the three, poison sumac causes the worst reaction." Luckily, he added, like poison oak, it is not a commonly found plant, and people aren't likely to come across it unless, like him, they spend a lot of time in the field. "Poison ivy," he added ruefully, is everywhere. "There are few places you don't see poison ivy."

How to identify poison ivy, oak and sumac
The rhyme "leaves of three, let it be" could describe a lot of plants, so you have to know what to look for to identify poison ivy, poison oak and poison sumac.