American Yellowwood (Cladrastis kentukea)

Yellowwoods are known for their brittle branches, which is how this genus was given its Latin name Cladrastis, meaning “fragile twigs.” As deciduous trees, they produce swaths of attractive flowers, making them a common choice to be planted in home gardens. The yellow wood derived from the tree is often used in making furniture and other woodcrafts. 

I. Appearance and Characteristics 

Cladrastis kentukea, the Kentucky yellowwood or American yellowwood (syn. C. lutea, C. tinctoria), is a species of Cladrastis native to the Southeastern United States, with a restricted range from western North Carolina west to eastern Oklahoma, and from southern Missouri and Indiana south to central Alabama. The tree is sometimes also called Virgilia.

Cladrastis kentukea is a small to medium-sized deciduous tree typically growing 10–15 meters (33–49 ft) tall, exceptionally to 27 meters (89 ft) tall, with a broad, rounded crown and smooth gray bark. The leaves are compound pinnate, 20–30 cm long, with 5-11 (mostly 7-9) alternately arranged leaflets; each leaflet broad ovate with an acute apex; 6–13 cm long and 3–7 cm broad, with an entire margin and a thinly to densely hairy underside. In the fall, the leaves turn a mix of yellow, gold, and orange.

The flowers are fragrant, white, produced in Wisteria-like racemes 15–30 cm long. Flowering is in early summer (June in its native region), and is variable from year to year, with heavy flowering every second or third year. The fruit is a pod 5–8 cm long, containing 2-6 seeds.

• Bark: Smooth gray, or light brown. Branchlets at first are downy, but soon become smooth, light yellowish green; later red brown, finally dark brown.
• Wood: Yellow to pale brown; heavy, hard, close-grained and strong. Sp. gr., 0.6278; weight of cu. ft., 39.12 lbs.
• Winter buds: Four in a group, making a tiny cone and enclosed in the hollow base of the petiole.
• Leaves: Alternate, pinnately compound, eight to twelve inches long, main stem stout, enlarged at base. Leaflets seven to eleven, broadly oval, three to four inches long. Wedge-shaped at base, entire, acute, terminal leaflets rhomboid-ovate. Feather-veined, midrib and primary veins prominent, grooved above, light yellow beneath. They come out the bud pale green, downy; when full grown are dark green above, pale beneath. In autumn they turn a bright clear yellow.
• Flowers: June. Perfect, papilionaceous, white, borne in drooping terminal panicles twelve to fourteen inches long, five to six inches broad, slightly fragrant.
• Calyx: Campanulate, five-lobed, enlarged on the upper side.
• Corolla: Papilionaceous; standard broad, white, marked on the inner surface with a pale yellow blotch; wings oblong; keel petals free.
• Stamens: Ten, free; filaments thread-like.
• Pistil: Ovary superior, linear, bright red, hairy, bearing a long incurved style.
• Fruit: Legume, smooth, linear-compressed, tipped with the remnants of the styles. Seeds four to six, dark brown.

Yellowwood won a Pennsylvania Horticultural Society Gold Award in 1994.

The Society of Municipal Arborists selected the yellowwood (Cladrastis kentukea or C. lutea) as its Urban Tree of the Year for 2015.

II. How to Grow and Care

Sunlight

Site this tree in a spot where it will receive at least part sun. Full sun is best to encourage flowering.

Temperature and Humidity

Hardy in Zones 4 through 8, Yellowwood tolerates both hot summers and cold winters well, although intense winter cold can reduce flowering in the spring. It’s a low maintenance specimen for your landscape that isn’t fussy about humidity.

Watering

For the first year in your garden, ensure the tree gets at least one inch of moisture per week via rainfall or irrigation. Known as a shade tree, the yellowwood works well in urban conditions thanks to its drought tolerance. This tree can handle dry spells as long as the roots have had a proper chance to spread through an initial growing season of regular watering.

Soil

Alkaline and acidic soils are both handled well by this tree as long as soil drains well. It can also tolerate the full range of soils from sand to clay.

Fertilizing

The yellowwood is one of the Fabaceae family members that does not utilize nitrogen fixation. You may need to fertilize it if tests show N levels are low and symptoms like yellowing foliage appear. Test to be sure as conditions like drought or overwatering have the potential to also cause yellow leaves.

Planting Instructions

Pruning

You should start pruning when the tree is young to help it keep good branch angles. The branches tend to grow close to each other and the wood can be brittle. Take these steps for adequate pruning:

• This tree tends to bleed sap from wounds made in the winter, so pruning should be performed in the summer or right after flowering is complete in the late spring.
• Make sure there is a central leader.
• Remove crossing branches and weak branch forks.
• Create strong branch angles that are wide to help keep the tree healthier and make sure new branches are farther apart.
• Be careful when working on this tree as the bark is thin and can quickly become damaged.

Propagation

Cuttings 

Propagate yellowwood trees from hardwood cuttings taken in the late fall or early winter after leaves drop and the tree is dormant. Here’s how:

• Take a 6- to 10-inch cutting that’s about the thickness of a pencil from the current season’s growth. It should be hard and woody and aim for the spot where the current season stem meets the prior season’s stem with a straight cut.
• Remove any softer. greener growth at the tip with an angled cut.
• Scrap off a bit of the bark at the straight cut to expose some of the cambium underneath.
• Dip this end into rooting hormone and insert 1/2 to 2/3 or your cutting into a pot filled with moist potting medium.
• Keep the potting medium moist. When you see new growth on the cutting, your new plant has developed roots and can be hardened off before allowing to grow outside.

Seed

Follow these steps to grow a yellowwood from seeds:

• First, scarify the seed to soften the coat by soaking it in warm water for 24 hours.
• Fill a plastic bag with moist peat moss or vermiculite and place seeds inside.
• Put the bag in the refrigerator for 60 days, checking the bag periodically to ensure it remains moist.
• Fill a pot with equal parts perlite, sand, and compost. Moisten thoroughly.
• Place the seed about 1/4-inch into the soil and cover.
• Put the pot in bright indirect light and mist the soil occasionally to keep it moist.
• After germination, continue to care for the seedling until it has developed several sets of leaves.
• Harden it off and plant it in the landscape.

Pests and Diseases

This low-maintenance, highly adaptive native tree does not have many pest or disease issues that crop up. It is possible, however, that verticillium wilt, cankers, rots, and decay could strike. The smooth gray bark may be susceptible to sun scald.

Maintenance

The low-forking branches sometimes form at weak angles which are prone to splitting and or breaking. While the tree is young, prune for strong branch structure in mid to late summer. Maintain a three inch mulch layer over the root zone to help maintain consistent soil moisture.

III. How to Get Yellowwood to Bloom

• Bloom Months

Yellowwood trees bloom in late May to early June in its native area.

• How long does Yellowwood Bloom?

The gorgeous floral display lasts about a week. While the flowers are a key feature of this plant, they can be erratic. Some years, the blossoms will be plentiful; other years, there will be sparse to none. New trees may not bloom for the first 8-10 years.

• What do Yellowwood Flowers look and smell like?

Long panicles of white wisteria-like flowers are about a foot long and cover a tree in a dense, spectacular display. The flowers are sweet, slightly vanilla-scented, and are most fragrant in the evening. They attract bees and Black Swallowtail butterflies.

IV. Types of Yellowwood Tree

The yellowwood belongs to the large Fabaceae (pea) family which includes species like wattles (Acacia spp.), silk tree (Albizia julibrissin), thornless honey locust (Gleditsia triacanthos var. inermis), and the eastern redbud (Cercis canadensis). In addition to the standard white flowered yellowwood tree, try this variety for pink blooms:

Cladrastis kentukea ‘Perkin’s Pink’: produces fragrant pink pea-like flower clusters in early summer; grows 25-40 feet tall

V. Uses and Benefits 

• Ornamental uses

Cladrastis kentukea is widely grown as an ornamental tree for its attractive flowers, and is locally naturalized in many areas of the eastern United States outside of its restricted native range. Kentucky yellowwood is recommended as one of the best medium-sized trees for cultivation as an ornamental plant in gardens. The only quality that is mentioned is a tendency of the trunk to divide very near the ground, as a multi-trunked tree. 

The smooth gray bark adds winter interest.

• Other uses

The name yellowwood derives from its yellow heartwood, used in small amounts for specialist furniture, gunstocks and decorative woodturning.

This plant has been marked as a pollinator plant, supporting and attracting bees and butterflies.