Why are some leaves red or purple all summer?

A copper beech or purple-leaf plum stays dark all summer, yet its leaves are still green inside. Here’s what’s going on.

Most leaves are green all summer, but a copper beech, a purple-leaf plum, or a ‘Bloodgood’ Japanese maple stays deep red or purple from spring to fall. Those leaves still have all the green machinery inside; it’s just hidden.

The pigment: anthocyanin

The red-to-purple color comes from pigments called anthocyanins, the same family that colors red apples, blueberries, and many fall leaves. In purple-leaved trees and shrubs, they build up in the leaves all season and mask the green of chlorophyll. The chloroplasts underneath are still there and still working (Wisconsin DNR). Anthocyanins absorb green and yellow light, which is why the leaves look red or purple to us (Dunedin Botanic Garden).

That’s different from fall color. In autumn, many trees make anthocyanins as their chlorophyll breaks down; purple-leaved plants make them while their chlorophyll is still at full strength. See why leaves change color for the fall side of the story.

Do purple leaves still make food?

Yes. They photosynthesize, though they may catch somewhat less of the light they need, which is why purple-leaved trees do best in full sun (Wisconsin DNR). Whether that slows them down is less clear-cut. Some studies find lower photosynthesis or slower growth in red-leaved forms, and others don’t. One study of red-leaved plums found lower rates in young and mature leaves but higher rates late in the season, when the red plants also held their leaves weeks longer (Lo Piccolo et al. 2018). A red-leaved pear made more food per leaf than a green one but grew less in its first year, because it had less leaf area early on (Tozzi et al. 2018).

What anthocyanins do for a plant

Anthocyanins aren’t just decoration. Researchers have found they can shield the food-making parts of a leaf from too much light, mop up harmful molecules that build up under stress, and help leaves tolerate drought, ultraviolet light, and other stresses (Gould 2004). Their exact job probably varies from plant to plant.

Where purple trees came from

Most purple-leaved trees in yards and parks are cultivated varieties, propagated from unusual plants that people noticed and kept.

• Copper beech. Purple-leaved forms of European beech arose as natural mutants in several parts of Europe and were known in Germany before 1488. Trees grown from seed vary in color, so the darkest ones are grafted to keep the color true (Royal Botanic Gardens, Kew; Woodland Trust).
• Purple-leaf plum. The cherry plum ‘Pissardii’ was found in Persia by the Shah’s gardener, M. Pissard, and sent to France in 1880 (International Dendrology Society).
• ‘Crimson King’ maple. A seedling of the Norway maple ‘Schwedleri’, raised in a French nursery and released around 1946 (Bean’s Trees and Shrubs).

Why some purple leaves turn green in summer

Some purple plants hold their color all season; others fade. The ‘Blireiana’ plum leafs out purple-red and turns green in summer (Cornell University). ‘Bloodgood’ Japanese maple stays red in summer except in hot, dry areas, where it can turn greenish red (NC State Extension), and Diabolo ninebark can turn dark green where summers are hot (Oregon State University). Sun matters: ‘Royal Purple’ smokebush colors best with six or more hours of sun a day (NC State Extension).

Red new leaves in spring

Many trees that are green in summer start out red. New leaves are often reddish until they build up chlorophyll, which takes light and warmth, so a cold, cloudy spring can keep them red longer (Northern Woodlands). Red-tip photinia’s red new growth lasts about two to four weeks before greening (Clemson Extension). Why trees bother is still debated. The red is often described as sunscreen, but a study of tropical trees found red young leaves were eaten less than green ones and concluded their main benefit was defense against plant-eaters (Gong et al. 2020).

What purple trees do in fall

Fall color varies. Copper beech leaves turn coppery (Woodland Trust). ‘Crimson King’ goes brown, dark maroon, or bronze (University of Florida IFAS). Purple-leaf plum doesn’t really change color (Franklin & Marshall College). ‘Bloodgood’ Japanese maple turns crimson to reddish-orange (NC State Extension), and ‘Royal Purple’ smokebush ranges from yellow to orange to red (NC State Extension).

A note before planting

Two popular purple plants are restricted in some states. Norway maple, including ‘Crimson King’, and Japanese barberry, including its purple forms, are on the Massachusetts Prohibited Plant List and can’t be sold there (UMass Extension), and Minnesota is phasing out most barberry varieties (Minnesota Department of Agriculture). Check your state’s invasive plant rules before buying.