Aesculus

( ESS - kew - luss )

At-A-Glance Information

Aesculus is a genus of deciduous trees and shrubs that are commonly referred to as Horsechestnut, Conker, or Buckeye. These plants need full sun and regular water, and they will attract hummingbirds and butterflies to the garden. Horsechestnuts can give an almost tropical look to the landscape, and produce flowers that appear in dense clusters at the ends of the branches, ranging in color from white to pink-red. Leathery fruit capsules appear in fall, and are called Conkers. Take care when planting these in your yard, as all parts of the plant are moderately toxic if ingested.

Fun Fact: The game of 'Conkers' is played using the seeds of Horse Chestnut Trees.
This fun, nature-centric, two-player childrens game uses Horse Chestnut tree seeds (called 'conkers') threaded onto strings. Players take turns, one dangling their conker on their string, and the other trying to smash it with their conker. A point is scored when a conker survives a hit, regardless of whether the conker is the attacking or dangling one. Some conkers will break at the first hit, others will survive multiple or all hits in the game.

There are various names for conkers that belie their strength. A none-er is a conker that does not have a score; it has not yet engaged in a round of the game. As each hit progresses, the conker gains points, and is called a one-er, two-er, three-er and so on as it continues to take hits without breaking. The conker that can take the most hits without breaking wins. This simple game became so popular that championships have been held since 1965.

Aesculus Species and Varietals

The genus Aesculus includes between 13 and 19 species, and is a member of the plant family Sapindaceae (Soapberry). This genus was named by Carl Linnaeus and the moniker is derived from the Roman word for 'edible acorn'. Although these trees are commonly called Horsechestnuts, they are not related to actual Chestnut trees (which are in the plant genus Castanea). The best growing zones for Horsechestnut trees vary by species. For more information, see the species descriptions below. To find your zone, click here: Zone Map.

Aesculus x arnoldiana 'Autumn Splendor' (Arnold Buckeye)
This tree grows best in USDA hardiness zones 3-9 or Sunset zones A2, A3, 1-9, and 14-16. This hybrid species is ornamentally attractive and cold-hardy. Individual trees grow to 35-40 feet tall and just as wide, with glossy dark green leaves that turn a brilliant maroon color in fall. Leaves are resistant to leaf scorch. Flowers are yellow with an orange-red blotch and bloom in clusters 8 inches long. These trees do best with some dappled shade, especially when young.

Aesculus californica (California Buckeye)
This can be grown as a shrub or a tree, and is native to the dry slopes and canyons below 4,000 feet in elevation in coast ranges and sierra foothills. This plant has a graceful habit and spreads to 10-20 feet tall and 30 feet wide, though it can get larger if given room. New foliage is apple-green and matures to rich green. These trees often have multiple trunks supporting its width. Cream-colored plumes of fragrant flowers appear in spring. Pear-shaped fruits form and split open revealing shiny brown seeds. Seedlings make unusual bonsai subjects. If given moderate water, the tree will hold its leaves until fall, and after they drop, the silvery trunk and branches make a great silhouette. This species is best in USDA growing zones 7-8 and Sunset zones 3-10 and 14-24.

Aesculus x carnea (Red Horsechestnut)
This tree grows best in USDA hardiness zones 5-8 and Sunset zones 1-10, 12, and 14-17 and grows to reach 40 feet tall and 30 feet wide at maturity. This tree has large, dark green leaves that form a dense canopy and ample shade. Flowers are soft pink to red. Varieties include 'Briotii' (with rosy crimson blossoms), 'Fort McNair' (with pink flowers that have yellow throats), and 'O'Neill Red' (with single bright red flowers).

Aesculus hippocastanum (Common Horsechestnut)
This tree grows best in zones 1-10, 12 and 14-17. This is native to European and gets to 60 feet tall with a 40 foot spread. Has a great, shade-heavy canopy area. Roots are strong and can break up paved surfaces, so plant away from walkways, driveways and other paved areas. Flowers bloom spectacularly in ivory with pink markings on one foot long plumes. Variety 'Baumannii' has double-blooms and doesn't set seed.

Aesculus pavia (Red Buckeye)
This shrub or tree grows best in USDA growing zones 5-9 and Sunset zones 2-9 and 14-24. It is native to the eastern United States and forms a bulky tree that reaches 12-20 feet tall and almost as wide. Leaves are glossy green and flowers bloom in narrow clusters and are bright red or orange-red. This is the best choice of Buckeye for humid climates.

Caring for Aesculus

Horse Chestnut trees can be grown from seed, but are typically purchased as starts from a reputable nursery. They require a space that receives full sun (though some species do better in partial shade in warmer climates). Ideally, they should receive between 6 and 8 hours of sun each day for best growth rate and flowering. The best time of the year to plant a Horse Chestnut tree is in early spring or fall. Ensure that soil is well-draining, rich in nutrients, and has a pH of between 5.0 and 8.0 (slightly acidic to neutral). If local soils are particularly poor in nutrients or do not drain well, make sure to amend the planting area with plenty of organic compost before installing. Apply a thick layer of mulch around the planting area to suppress weed growth, maintain consistent soil temperatures, and to increase soil moisture retention. Ensure that mulch coverage stops a few inches away from the trunk. Water new plantings regularly as they establish a strong root system in their new home. When mature, these trees are relatively drought-tolerant, but will benefit from deep waterings every few months, especially during the warmer times of the year.

Fertilize yearly in early springtime with an application of 2 inches of compost around the base of the tree. Pull mulch back before applying, work the compost gently into the soil a bit, water it in, and then replace mulch. As they are getting established, they will benefit from some support from staking, especially if they are in a high-wind area. Make sure to remove stakes as the tree grows to avoid choking issues. Prune trees in late winter or early springtime, thinning out crossing, damaged, or diseased branches and foliage and opening up the canopy to increase airflow. Heavy prunings are not recommended, as they can significantly stress out the tree. Remove any suckers that form at the base of the tree. Another pruning can be completed in early summertime for shaping and sucker removal. In wintertime in areas that experience frost, protect young trees with burlap or frost cloth. Aesculus trees, if properly cared for, can live for decades to centuries in the landscape (lifespan is somewhat species-dependent).

Growing Aesculus in Containers
Although Aesculus trees are generally considered large and unfit for container growing, dwarf cultivars can do well long-term in these growing conditions. Examples of Horse Chestnut cultivars that do well in containers include Aesculus parviflora and Aesculus glabra 'Lavaburst'. Choose a very large container with a diameter of 16 inches or more (at least four to six inches larger than the current size of the root ball). Ensure that the container has adequate drainage holes in the bottom to allow for excess water to drain away. Fill the container with a well-draining potting mix, and plant the tree, ensuring that the top of soil is about an inch below the rim of the container. Apply a dressing of mulch around the base of the trunk, ensuring that it stays a few inches away to avoid crown rot issues. Container-grown Aesculus will need more frequent waterings than those grown in the ground; as a general rule of thumb, water whenever the top two inches of soil feels dry to the touch. Every few years, pot up the tree to a larger size to accommodate its growing root ball. Fertilize yearly in early springtime with an application of 2 inches of compost around the base of the tree. Pull mulch back before applying, work the compost gently into the soil a bit, water it in, and then replace mulch.

Propagation

Aesculus can be propagated either by using cuttings or from seed. To ensure that your propagated plant will have the exact same qualities and characteristics as the parent plant, use the cutting method. If you would like to experiment and see what plant you can grow from seed, just keep in mind that it will not necessarily be the same as the parent plant. To propagate your Horse Chestnut, follow the instructions below:

Propagating Aesculus by Cutting

Gather your materials. You'll need a clean, sanitized, sharp pair of garden shears, rooting hormone, a pencil, and a 3-inch pot filled with well-draining compost or other organic material. If you don't have any potting medium on hand, you can make some. A good substrate for propagation of cuttings is a light blend of 1/3 pasteurized soil or compost, 1/3 sand, vermiculite or perlite, and 1/3 coconut coir or peat moss.
Choose the plant that you'd like to propagate. Ensure that this plant has had a good watering the night before, as this will improve the chances your cutting will take root. Cuttings can be taken either in spring (for softwood cuttings) or fall (for hardwood cuttings). Cut one or more 4-6 inch portion(s) of the end of a non-blooming branch and strip the leaves off of the lower half. Make a hole in your planting medium by pushing your pencil into the soil. Dip the end of the stem in rooting hormone, and plant it in your pot. Fill in soil around the cutting to support it, but do not pack it down too much.
Moisten with water, and ensure that the pot has good drainage. Your pot should be kept in warm, sunny conditions indoors (and not in direct sunlight). Water whenever your soil is dry under the top layer. After about four to six weeks have passed, your cutting should be rooted. When you notice new growth, you'll know you have roots.
Keep your new plant in an area with bright light (though not direct sunlight) and temperatures between 55 and 70 degrees Fahrenheit. Ensure that the soil stays moist, but not wet, until the next spring, when you can transplant it to its new home in the landscape. Ensure that the last frost of the year has occurred before doing this, as it can seriously damage your new tree, especially if it is a warmer-climate species.

Propagating Aesculus by Seed

Keep in mind that the plant you get from seed will not necessarily mimic all of the characteristics of the parent plant. To propagate by seed, you will need a paper bag, and a pot filled with well-draining compost or other organic material. If you don't have any potting medium on hand, you can make some. Just mix together equal parts (by volume) moss peat and horticultural-grade sand or grit sand. You will also, after a few weeks, need individual 3-inch pots for each seedling that grows.
Harvest your seeds. The seed pods on these trees are pretty easy to spot, they're large, green-yellow and dangle from the branches, left behind after flowers have faded. Once they have dried out and begun to split open, you can harvest the seeds inside. Cut them off of the plant, placing them in a paper bag. Harvest as many as you can, and place the paper bag in a dry, cool, dark area. They will begin to split open and release the seeds contained inside. Once they have been released, take them out of the bag and put them in a container so you can work with them. Ensure that you use your seeds soon after they are harvested; if they are left out too long to dry, they develop a wrinkly texture and lose viability. To check to see if a seed is viable, place it in a glass of water. If it sinks, it is viable, and if it floats, it is not. In nature, most seeds will germinate in early springtime, though California Buckeye seeds germinate just after winter rains have begun (which is usually in November).
Buckeye seeds have a hard coating that protects them, but this coating also makes it a little difficult to get them to germinate. We can help the process along by scarifying them with sandpaper (or simply rubbing them against an abrasive surface). Do this with as many seeds as you can; they will probably not all sow, and the ones that do will be of varying characteristics. The more seeds you plant, the more chances you will have to get the resulting shrub you want. Place your seeds in lukewarm water for 48 hours.
Water your prepared pot filled with soil, and ensure that good drainage is occurring. Place your seeds in your prepared pot or seed tray about a quarter inch to a half-inch under the soil surface. Cover them with soil and water. Place your pot of seeds in a warm area that receives bright, indirect light. A greenhouse is the most ideal climate, but a planting tray inside a window will do as well. Germination usually takes 21-28 days from sowing. Once you begin to see seedlings sprout and they are able to be handled, transplant them to their own individual pots, filled with the same potting medium as described in Step 1. Loosen the seedlings from the potting medium by digging down, gently working the seedling up until it is out. Then, replant the seedling in its individual pot.
Keep the seedling's potting medium moist, but not wet, as it develops. Keep your new plant in an area with bright light (though not direct sunlight) and temperatures between 55 and 70 degrees Fahrenheit. Ensure that the soil stays moist, but not wet, until the next spring (after the danger of frost has passed), when you can transplant it to its new home.

Horse Chestnut seeds, called Conkers, begin light green and mature to a dark brown. They will fall from the tree when ready.

Seeds within the pods on the tree will begin to emerge as the pod splits open.

Problems and Pests

Horse Chestnut trees are susceptible to developing issues with the following pests and diseases:

Aphids
Leaf Spot
Bleeding Canker
Powdery Mildew
Horse Chestnut Scale

For more information on these issues, including how to treat and prevent them, consult the following links:

Pests & Pest Management

Plant Issues

Gallery

Aesculus x carnea 'Briotti'

Aesculus x carnea 'Fort McNair'

Aesculus x carnea 'O'Neill Red'

The seeds are called 'Conkers'.

Use these trees in areas that need shade.

The 'Anne Frank Tree', which gained notoriety for being the tree mentioned in The Diary of a Young Girl, was a Horsechestnut. Sadly, the tree fell in 2010 during a storm.