"Leaves of three, let them be" is a useful saying to warn people away from touching poisonous plants. Not all plants that have three leaves are poisonous. Still, many poisonous plants do indeed have leaves that are grouped in threes. Other plants that have three leaves range from weeds to houseplants. Trees with three leaves are hard to find, but some deciduous trees have single leaves with three distinct sections, or lobes.
The shamrock plant (Oxalis regnelli) is a tender perennial often grown as a houseplant. Oxalis regnelli features three triangular-shaped leaves attached at the corners to a thin stem. The small, creamy-white flowers bloom in early spring and into the summer if given proper care, according to the University of Arkansas. Shamrocks thrive in bright but indirect light. Water these plants when the soil dries out slightly to the touch, and fertilise once a month with a houseplant fertiliser during the summer.
White clover (Trifolium repens) is a perennial plant that often invades lawns. The oblong leaves are grouped in threes. Each one is called a leaflet, and is connected at the centre to a thin stem. White clover stems can reach 16 inches in height. Flower heads are round and covered with between 20 and 40 tiny white flowers, according to Virginia Tech University. White clover is sometimes planted for animals to forge on.
Poison ivy (Toxicodendron radicans) grows as a shrub, a climbing vine or a trailing vine, according to the University of Missouri. The leaves are made of three leaflets that are attached to the stem in an alternate pattern. The leaflets may be toothed on the margins or smooth. The green leaves turn red, orange and yellow in the fall. All parts of this plant are poisonous to the touch and will cause a painful or itchy rash if they come into contact with the skin.
The Amur maple (Acer ginnala) is a small, cold-hardy deciduous tree. It usually grows on more than one trunk, according to the University of Connecticut. Amur maple trees have three-lobed leaves, with the centre lobe being much larger than the two flanking lobes. In the spring, this tree blooms with small but fragrant yellow flowers. In the fall, the tree puts on a showy display of colours. Amur maples will grow in full sun or partial shade and do best in cooler climates.
Sugar maple trees (Acer saccharum) grow primarily in the northern and northeastern areas of the United States and into Canada. They are known for their sweet sap and brilliant fall colour. Sugar maple trees have three lobed leaves. Sometimes they can have two more, smaller lobes at the base of each leaf. Sugar maples can get quite large -- to over 100 feet tall in the wild -- and grow best in moist, well-draining soils. They are hardy to (USDA) growing zone 3.