Why new leaves come out red

Watch an oak, a red maple, a sweetgum or a black cherry break bud in April and the first leaves are rarely green. They come out bronze, pink, wine-red or nearly purple, and they stay that way for a week or three before settling into the green you expect. New growers often read this as a problem: a nutrient deficiency, cold damage, a disease. It is none of those. It is one of the more elegant tricks a tree has.

The pigment is anthocyanin, and it is a sunscreen

The red color comes from anthocyanins, the same water-soluble pigments that color blueberries, red cabbage and autumn leaves. A brand-new leaf is thin, its cuticle is unfinished, and its chloroplasts are still being assembled. It has very little chlorophyll and almost none of the protective machinery a mature leaf uses to dump excess light energy as heat.

That leaves the young leaf exposed. Full spring sun delivers far more energy than an immature photosystem can use, and the surplus damages the light-harvesting proteins, a process called photoinhibition. Anthocyanins sit in the outer cell layers and absorb strongly in the green and blue part of the spectrum, cutting the light that reaches the developing chloroplasts underneath. They also mop up the reactive oxygen molecules that excess light produces. In short, the leaf builds a temporary red sunshade, works underneath it until the green machinery is ready, then dismantles the shade.

How you can tell that is what is happening

• The flush is strongest on the sunniest leaves. Look at the top of the crown versus the shaded interior of the same tree; the interior leaves emerge greener.
• It fades on schedule. As chlorophyll accumulates and the cuticle thickens, anthocyanin production stops and the red is diluted, usually within two to three weeks of leaf expansion.
• Seedlings do it more than mature trees, and trees in open sun do it more than trees in the woods. Expect it on the seedlings and saplings you plant out in full sun.
• The same thing happens across the world’s tropics, where many trees produce limp, red or white new leaves that green up later. Botanists call it delayed greening.

There is a second idea, not exclusive of the first, that red young leaves are less visible or less attractive to leaf-eating insects, many of which see poorly in red. Both mechanisms can operate at once. Either way, the tree is protecting its most expensive new tissue during its most vulnerable week.

Not the same as fall color

Autumn anthocyanins are made for a related reason as the leaf is being shut down. The tree dismantles chlorophyll to recover nitrogen and other nutrients, and during that salvage operation the leaf is again exposed to more light than it can use. Red pigments protect it long enough to finish the job. Spring red protects a leaf being built; fall red protects a leaf being taken apart. If it is the autumn show you are after, the catalog can be filtered to plants known for fall color.

When to actually worry

A red or bronze flush on healthy, fully expanding leaves needs nothing from you. Do not fertilize it, spray it or water it more. Look instead for the signs of real trouble, which have a different character:

• Leaves that stay small, cupped or crinkled after they should have expanded.
• Scorched or brown margins, which point to water stress or root damage; on a newly planted tree, check the watering schedule first.
• Wilting at midday that does not recover overnight.
• Red or purple color that persists into summer on a tree that is not a purple-leaved cultivar, which can indicate phosphorus deficiency or root problems.

If the leaf is expanding normally and the color is fading week by week, the tree is doing exactly what it should. Most of the real problems on young trees trace back to planting day, not to the color of the leaves.