Which tree has thorns? Look at where they grow

Thorns, spines and prickles

Botanists use three words for the sharp points on trees, and the position of the point on the twig shows which one you have. According to NC State University's plant defense exercise, a thorn is a modified stem, borne immediately above a leaf (or where a leaf would be), so it can branch like a shoot; a spine is a modified leaf or stipule, borne in the leaf's position below the bud; and a prickle is a modified hair, scattered over the stem with no particular relation to the leaves. NC State lists hawthorns and honey locust as thorny, black locust as spiny, and devil's walking stick and Hercules' club as prickly. In everyday speech all three are "thorns", and this page uses the word that way in its headings.

Thorns on the trunk: honey locust and silk floss tree

Honey locust is the tree people mean when they say "thorns growing out of the trunk". Texas A&M Forest Service's Trees of Texas entry describes strong, sharp, brown thorns, straight or branched, on the trunk, branches and twigs, which form on one-year-old wood and stay for many years. NC State's honey locust profile gives stout thorns up to 3 inches long with one or three branches. The leaves are feather-like, 5 to 10 inches long with 15 to 30 leaflets, and the dark brown pods are 10 to 18 inches long, flat and often twisted. Thornless forms are widely planted along streets, so a smooth honey locust in a parking lot is no surprise. The locust page compares honey and black locust in detail.

Silk floss tree is the trunk-thorned tree of the frost-free South. UF/IFAS's fact sheet describes an unusually thick trunk that stays green even on older trees and is covered with sharp spines, as are the young, green branches. In fall, usually October, it flowers pink and white on nearly bare branches, then hangs 8-inch pods full of silky floss. It grows in USDA zones 10A to 11.

Thorns or spines on the branches and twigs

Black locust has the smallest weapons here: Texas A&M Forest Service's black locust entry describes a pair of sharp, strong spines a quarter to half an inch long at the base of each leaf. Leaves are 10 to 14 inches long with 11 to 19 oval leaflets, and the bark is thick, deeply furrowed and light brown. In spring it hangs fragrant white flowers.

Hawthorns are a large group; Texas A&M Forest Service counts 30 or so species, and its hawthorn entry describes small trees, usually to 20 feet, most with long, straight thorns on the twigs, simple toothed leaves 2 to 3 inches long, dense flat clusters of white flowers in spring and round red or yellow fruit a quarter inch to 1 inch across. See the hawthorn page.

Osage orange (bois d'arc, hedge apple) is the answer to "tree with thorns and green balls". Its Trees of Texas entry describes crooked, interwoven, thorny branches, twigs armed with stout straight thorns that bleed sticky, milky sap when broken, glossy leaves 3 to 5 inches long, and a wrinkled green fruit 4 to 5 inches across. It was planted widely for hedgerows before barbed wire.

Wild Callery pear is the thorny form of the Bradford pear. NC State Extension's Callery pear publication explains that the planted cultivars are thornless but their wild offspring have large thorns, up to 4 inches long, on branches and stems, and form dense thickets with an irregular, multi-trunked shape. White flowers that smell bad in early spring confirm it; the Bradford pear page covers its spread and state sale bans.

Russian olive is easiest to name by its color. NC State's Russian olive profile describes a thorny large shrub or small tree up to 30 feet, with narrow leaves 1.5 to 4 inches long covered in silvery scales, young branches that are silvery and occasionally thorny, fragrant silvery-white to yellow flowers from April to June, and half-inch, olive-like fruit that is silver in late summer and turns red in fall. It is listed as invasive by the NC Invasive Plant Council.

Wild plums form thorny thickets. NC State's American plum profile describes thorn-like side branches, twigs that sometimes carry thorny side branchlets, white flowers from March to April before the leaves, and round red-purple plums about an inch across in July and August.

Trifoliate orange is a hardy citrus, first grown as a rootstock for grafted citrus trees. NC State's trifoliate orange profile describes bright green twigs with numerous long, sharp green thorns, leaves of three leaflets on a winged stalk, white flowers in mid-spring and dull lemon-yellow fruit like a small, very sour orange. It is listed as invasive in North Carolina by the NC Invasive Plant Council.

Thorns all over it: devil's walking stick and Hercules' club

Both of these are often called prickly ash and Hercules' club, which makes the names confusing; the leaves sort them out.

Devil's walking stick is often a single stem up to 30 feet tall with a flat-topped crown. Texas A&M Forest Service's entry describes light gray bark armed over the entire surface with stout, sharp prickles, and enormous twice-divided leaves 3 to 4 feet long with prickles along the leaf stalk. In midsummer each plant raises a panicle of small white flowers about a foot across, followed by tiny black fruit.

Hercules' club carries prickles of a different shape. The Forest Service's Hercules'-club entry describes light gray bark covered with knobby, cone-shaped prickles "shaped somewhat like crocodile teeth", up to 2 inches across and 1 inch tall, and once-divided leaves 5 to 8 inches long with 7 to 17 toothed leaflets and sharp prickles along the stalk. Chewing the twigs or inner bark numbs the mouth, which gave it the name toothache tree.

Texas and the Southwest: mesquite, huisache and retama

In Texas, most thorny trees have feathery, finely divided leaves. Honey mesquite, one of the most common trees in the state, has twigs and branches armed with stout, straight thorns up to 2 inches long, leaves with two long, narrow-leafleted branches, and pods 4 to 9 inches long, according to the Forest Service's mesquite entry. Huisache has a pair of straight thorns up to 2 inches at the base of each leaf and fragrant orange-gold puff-ball flowers in spring (entry). Retama has green twigs and thorns up to 1 inch. Their flowers are compared in trees with yellow flowers.

In Britain: hawthorn and blackthorn

In British hedgerows the two thorny trees are hawthorn and blackthorn. The Woodland Trust describes blackthorn as spiny and densely branched, with twigs that form straight side shoots which turn into thorns, snow-white flowers in early spring and purple fruits called sloes. The differences are laid out in hawthorn vs blackthorn.

A photo of the thorns and a leaf

Thorns alone rarely name a tree, but a photo that shows the thorns, a leaf and any fruit together usually does. Tree Identifier names a tree from one photo; honey locust and black locust are in its Collection, and it also names trees outside it, such as Osage orange. It shows no confidence score, so check the result against where the thorns sit, and use the follow-up chat to ask how a thorn differs from a spine on that tree. In winter, when the leaves are gone, the winter tree identification guide helps with twigs and buds.