There are many plant and weed species common to North America that can cause hives in those with allergies. Antihistamine pills, topical creams, and lukewarm washes can help relieve hives symptoms.

A person can develop hives — an itchy raised rash — when they come into contact with a trigger, which can be synthetic or natural. This then induces a histamine reaction in the skin, leading to an itchy rash.

Poison ivy may be the plant people most commonly think of in relation to hives, but several other weeds can also cause skin reactions, including ragweed and poison sumac.

In this article, we will explore which types of plants can cause hives in a person, what signs and symptoms contact with these plants may cause, and the treatments for hives from plants and weeds.

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A common plant in the United States, poison ivy typically grows in the Midwest and eastern states.

If a person touches poison ivy, they risk developing a severe rash that can blister and itch for days or even weeks. The rash may take 1–2 days or up to a week to appear following exposure. It usually takes 1–2 weeks to go away.

All parts of the plant contain an oil called urushiol. It is this oil, also present in poison oak and poison sumac, that causes the rash. Even when dead, the leaves and stems can cause a rash.

Poison ivy usually grows as a vine in the United States, but it can also be a shrub. Typically, solid green, pointed leaves hang from its stem in groups of three. It may also produce yellow-green flowers and off-white berries in spring and early fall.

A person may have a ragweed pollen allergy. Ragweed grows wild in nearly all parts of the United States, especially in the East and the Midwest, in both suburban and rural areas.

The plant begins to grow in spring and usually flowers around mid-August. Warm weather, low humidity, and breezes encourage ragweed to release its pollen.

A person can develop hives, among other allergy symptoms, from ragweed pollen, either by touching the plant or from coming into contact with the airborne pollen.

Ragweed can cause itchy, red streaks on the skin 24–48 hours following exposure. It can be painful and develop into blisters, lasting 2–3 weeks, but usually goes away on its own.

Ragweed is in the genus Ambrosia, from the aster family. Common ragweed looks like a fern, with a series of thin leaves positioned opposite each other. It tends to grow about 3.5 feet tall, and people often see it in yards and gardens.

Giant ragweed grows 3–17 feet tall and has 3–5 lobes of leaves. It looks less fern-like.

Poison oak can also cause a severe rash if a person touches it. Like poison ivy, it is a common plant in the United States. However, a person is more likely to find poison oak in the Western states.

The same oil — urushiol — causes an itchy rash when a person comes into contact with poison oak and poison ivy.

Poison oak leaves look similar to the leaves of poison ivy, except poison oak grows more like a shrub than a vine.

As with poison ivy and poison oak, exposure to poison sumac can cause a severe rash within 1 day to 1 week. The rash may last up to 2 weeks.

People more commonly find poison sumac in the Midwest and eastern states.

A person can identify poison sumac by its leaves arranged on each side of a branch, with the plant taking the form of a shrub rather than a vine.

Giant hogweed belongs to the Heracleum plant family, which grows in Europe, Asia, and North America. It has hollow, reddish-purple stems and large, umbrella-shaped clusters of white flowers.

Plant experts consider giant hogweed to be the most invasive and hazardous plant in this family. It is much larger than other plants in its family and can grow as big as 12–15 feet tall and 3 feet wide.

Heracleum sap, stems, leaves, and fruits contain toxic chemical compounds known as furanocoumarins. Skin becomes more sensitive to UV light when it comes into contact with Heracleum.

This can cause phototoxic dermatitis, which is a sunlight-induced inflammatory skin reaction. Some people experience mild redness and irritation while others get blisters, burns, scarring, or eye injury.

A person’s skin may react within 24 hours of exposure to giant hogweed, and peak sensitivity occurs within 3 days of contact. Some people experience ongoing symptoms for weeks to months.

Wood nettle, also called stinging nettle, is a member of the Urticaceae plant family.

It has a single, slightly zig-zagging stem and stinging hairs. Its oblong, pointed leaves are on long stalks and have coarse teeth. The plant also produces small clusters of light green flowers.

Wood nettle grows 3–5 feet tall in damp, rich, low woodlands and swamps as well as on riverbanks and roadsides.

When a person comes into contact with wood nettle, its stinging hairs behave like tiny syringes and inject alkaloids, which are the plant’s own natural pesticides, and other toxins into the skin. The skin then reacts with histamines, leading to a burning, itching feeling.

If a person develops hives from plants, they can prevent infection by avoiding scratching. Hot baths and showers can worsen the itching, but short, lukewarm or cool baths can provide relief.

A person can also try topical treatments to reduce the itching, such as:

  • colloidal oatmeal baths
  • calamine lotion
  • hydrocortisone cream
  • applying a cool, wet washcloth directly to the hives

Antihistamine tablets may also help with this, but antihistamine creams may make the hives worse.

A person should contact a doctor if the hives cover a large area of their body or if they are unable to relieve the itching themselves.

Several different weeds can cause an itchy rash, which can sometimes be severe and last a long time. These include poison oak, poison ivy, hogweed, and several others.

Different weeds tend to grow in different parts of the United States. For instance, poison ivy is more common in the Midwest and eastern states, while poison oak is more common in the western states.

A person can prevent the hives from becoming infected by avoiding scratching. Treatments to reduce the itching may include cool baths with colloidal oatmeal, calamine lotion, and antihistamine tablets.

A person should contact a doctor if the rash covers a large area of their body or if they cannot relieve the itching.