What are leaf buds?

Look closely at a bare twig in winter and you’ll see small, hard bumps along it. Those are buds, and each one holds a shoot, a set of leaves, or flowers that are already made and waiting for spring. Here’s what’s inside a bud, when trees make them, and how to tell one kind from another.

What’s inside a bud

A bud is an embryonic growing point wrapped up for safekeeping. Botanists sort buds by what they’ll become: a leaf bud grows into a leafy shoot, a flower bud opens into flowers, and a mixed bud produces both (College of St. Benedict and St. John’s University). At the center of a leaf bud is the shoot’s growing tip, surrounded by tiny leaves. Slice a bud lengthwise and look with a magnifying glass, and you can often see the miniature green leaves folded inside (Northern Woodlands). Each kind of tree folds them in its own set pattern (Northern Woodlands).

When buds form

Buds aren’t made in spring. A tree builds next year’s buds during the summer and fall before, one above each leaf (University of Wisconsin–Green Bay). By the time the leaves drop in autumn, the buds are finished (Northern Woodlands).

Shortening days are the signal for many trees. As days shorten in late summer, trees stop growing and set a bud around the growing tip, and the tissues inside go dormant for the winter. The day length that triggers this comes earlier the farther north a tree grows, and a few trees, such as apple and pear, respond more to temperature than to day length (Azeez and Sane, Plant Signaling and Behavior). Before winter, trees also pull water out of their buds, which helps protect them from frost damage (Northern Woodlands).

On some trees the bud holds the whole season’s leaves. On others it’s just a start. Leaves that begin forming one summer, wait in the bud over winter, and finish the next spring are called preformed; in oaks and sweetgum, their detailed shape is already visible inside the bud. A small catalpa bud can hold about 10 leaf beginnings, close to the number of full-grown leaves at the end of the season (Arnold Arboretum). Beech and white ash put out all their leaves at once in spring, while paper birch keeps making and opening new buds through the summer (Northern Woodlands).

Where buds grow on a twig

Most twigs end in a terminal bud, which continues the twig’s growth next year. Along the sides are lateral buds, one in the angle just above every leaf, so in winter you’ll find each one sitting right above a leaf scar. Some trees add accessory buds, extra buds stacked above or beside a lateral bud (College of St. Benedict and St. John’s University). Butternut’s side buds are often stacked one above another (University of Vermont), and red maple can have extra buds beside its side buds (Virginia Tech).

A few trees have no true terminal bud. In elms, the tip of the twig dies back to the nearest side bud, which then looks like an end bud (College of St. Benedict and St. John’s University). Basswood and paper birch also lack a true terminal bud (Virginia Tech, Virginia Tech).

The arrangement of buds is one of the quickest ways to identify a tree in winter. Buds in opposite pairs point to a small group: maples, ashes, dogwoods, the honeysuckle family, and horse chestnuts and buckeyes. Students remember them as “MADCap Horse” (College of St. Benedict and St. John’s University). Most other trees, such as elms, have buds that alternate up the twig (Northern Woodlands).

Leaf buds vs. flower buds

On many trees you can tell the two apart at a glance: flower buds are usually larger and rounder than leaf buds (College of St. Benedict and St. John’s University). Apple and pear fruit buds are plump, round, and downy, much fatter than the slender, pointed buds that make leaves (Royal Horticultural Society). Flowering dogwood is a clear example: its flower buds sit at the twig tips, wider than they are long and flattened on top, while its leaf buds are small and slender (University of Illinois). Red maple’s flower buds come in clusters (University of Nebraska–Lincoln), and its red flowers open before the leaves (Virginia Tech), so they can tint a hillside before any leaves appear.

Bud scales, and buds without them

Most buds are wrapped in bud scales, which are modified leaves, often coated with a sticky substance, that shield the tender tissue inside through the winter. When the bud opens in spring, the scales fall away and leave a ring of scars on the twig (Nebraska Extension). Botanists describe three main kinds of bud (University of Wisconsin–Green Bay):

Tulip tree buds have two scales that meet edge to edge and are shaped like a duck’s bill (Virginia Tech); bitternut hickory’s yellow buds work the same way (Woody Plants of Ohio). Witch-hazel and pawpaw have naked buds with no scales at all; witch-hazel’s is really a tiny folded leaf (Virginia Tech, Virginia Tech). Some buds add other defenses. Horse chestnut’s end bud is shiny and sticky (Virginia Tech), balsam poplar’s buds are resinous and fragrant (Virginia Tech), and magnolia buds are covered in gray fur (Arnold Arboretum).

Buds of common fall-color trees

Descriptions come from Virginia Tech’s dendrology fact sheets for sugar maple, American beech, tulip tree, white ash, and paper birch; the University of Nebraska–Lincoln for red maple; the Wisconsin LEAF program and Northern Woodlands for oaks; and the University of Illinois for dogwood. A few more worth knowing: basswood’s buds are red, plump, and lopsided, with just two or three visible scales, on zigzag twigs (University of Vermont); quaking aspen’s are dark brown and look varnished (Maine Forest Service); and butternut has a large, fuzzy, cream-colored end bud (Northern Woodlands). For the leaves these buds become, see the leaf guide.

Backup buds

Not every bud opens in spring. The end bud of a twig releases a hormone that holds back the buds below it, and if that bud is lost, a side bud takes over. Basswood has no true end bud, so its top side bud always serves as the leader (Northern Woodlands). Side buds that don’t grow out the following spring become latent buds (College of St. Benedict and St. John’s University), and they can wait for many seasons as a kind of life insurance (Nebraska Extension). Some end up buried under the bark and can later send out sprouts straight from the trunk, and aspen roots carry buds that send up whole new trees (Forest Encyclopedia Network).

Those reserves are what let a tree recover from a hard spring freeze. In Nebraska, a magnolia whose new leaf buds were killed by an April 2007 freeze had leafed out again by late May (Nebraska Extension). After a May 2011 frost in a German beech forest, trees began regrowing leaves 19 to 38 days later (Menzel and others, Frontiers in Plant Science). There’s more on freezes and other weather in how weather affects spring buds.

Buds keep a record

Each spring, the opening end bud leaves a ring of scale scars around the twig. Count the rings back from the tip and you know how old that part of the twig is; the distance from the tip to the first ring is how much it grew last year (University of Illinois Extension). More in what trees do in winter.

Winter food

Packed with nutrition, buds feed a lot of wildlife through the winter. Ruffed grouse eat aspen and birch buds; moose and deer browse maple buds; and snowshoe hares, squirrels, wild turkeys, purple finches, and pine grosbeaks eat buds too (Northern Woodlands).