Also known as wolfsbane or aconite, monkshood (Aconitum napellus) is an attractive yet highly toxic flower. The blossoms have a unique shape and deep purple hue, and the foliage is reminiscent of a buttercup's. But the beauty of monkshood doesn't even begin to outweigh the negatives—all parts of this plant are dangerous, including the roots, stems, leaves, and flowers. The pre-flowering leaves, seeds, and roots are especially toxic.

"Aconitum can cause nausea and vomiting if ingested," says horticulturist Christine Froehlich. The toxins disrupt sodium channels in humans and animals, causing a string of serious and potentially fatal gastrointestinal, nervous system, and cardiac ailments. The toxins can even be absorbed through the skin, so avoid touching monkshood if you see it in a garden or the wild.

Oleander (Nerium oleander) is another plant that may be more trouble than it's worth. "I can’t think of a more beautiful, popular, and deadly garden favorite than the oleander," says Jane Gates of Gates & Croft Horticultural Design. She adds that oleander does have utilitarian uses as a hedge, focal point, or backdrop. "Yet only one or two small leaves eaten by a child or pet can be lethal," Gates warns.

"Sometimes fallen leaves can blow into a neighbor’s yard and poison a neighboring pet or stock animal. This is one tempting garden plant that isn’t worth the risk to welcome into your garden." The entire shrub is poisonous (especially the leaves), and ingestion can lead to vomiting and cardiac issues.

English ivy (Hedera helix) has lovely leaves that climb fences and trellises, but it's toxic and invasive. "Because it is such a quick, easy ground cover that can blanket large areas quickly, it is often used to fill spaces in landscapes all over the country," says Gates. "But ingested leaves and berries are poisonous." When eaten, English ivy can cause vomiting, diarrhea, or a coma in people and animals. "Some sensitive people may also develop a skin irritation from touching leaves or stems," says Gates. "Think twice before choosing English ivy for your garden."

Lily-of-the-valley (Convallaria majalis) is a lovely ground cover with delicate white blossoms and bright red berries. It's easy to grow, fragrant, and works well in landscaping since it spreads easily (sometimes too easily). But the problem is that this plant is significantly toxic. All parts of the plant can cause abdominal pain, seizures, an irregular or slow pulse, and disorientation.

Sometimes even small plants are trouble. "One toxic but pretty perennial is Ruta graveolens, commonly known as rue," says Froehlich. Don't be lured in by the plant's delicate yellow blossoms. "The lacy gray foliage is attractive, but it emits several toxins, particularly furanocoumarins. Coming into contact with the plant by touching, brushing against it on a hot day, or pruning causes severe skin blistering and rash that looks like a burn," she says.

With its upright, bell-shaped flowers, there's no question that foxglove (Digitalis purpurea) is gorgeous, but all parts of the plant are poisonous. "Foxglove, another poisonous perennial, contains digitalis that can affect the heart," says Froehlich. Consuming the leaves or flowers of this plant can slow down the heart rate or cause it to become irregular.

Gardeners in warm climates might be attracted to the stunning pendulous blooms of Angel's trumpet (Brugmansia). Sadly, it's very toxic, and not just when ingested—skin contact with any portion of the plant is also a major no-no when dealing with this plant. Angel's trumpet can cause seizures, coma, hallucinations, and vision problems. Even the odor of the plant can cause headaches and nausea.