A large rounded deciduous tree that grows to around 12m tall and 8m wide.
Flowers and foliage
Small non-showy greenish-white flowers are borne in spring followed by dark blue fruits but these are often both quite inconspicuous. Leaves are ovate and slightly toothed and bright green in spring. In autumn they put on a spectacular show when they turn brilliant orange-scarlet before falling.
Easily grown in average medium to wet acidic soils in full sun to part shade. Shelter from dry winds until established. Tolerates poorly-drained soils (clay) and can grow in standing water. On the other end of the spectrum, it also tolerates some drought and adapts to some dryish soils. Although slow-growing it still needs to be sited in an area which affords plenty of room for future growth, particularly since it is difficult to transplant successfully.
Preparation for planting
Always choose healthy well grown plants and plant after autumn rains as the soil is moist and warm and allows plants to become established before winter. This enables them to withstand dry periods during the following summer. Young plants require thorough watering during dry periods over the first two or three years mulching helps to conserve moisture and suppress weeds. Planting success is often improved on clay soils by adding extra topsoil and raising beds. Incorporate coarse sand, bark, compost or other organic material to improve soil structure.
Before planting ensure the root ball is saturated and remove the planter bag or pot with minimal root disturbance. Trim any broken roots and plant at the same level as in the container. Dig a hole twice the diameter of the root ball and firm in and water once planted. Make sure plants are watered well until established if planting in a drier period. Plant with some general slow release fertiliser and then every spring apply an organic based fertiliser such as blood and bone at a handful per square meter as new growth begins. The worst fate for a tree is to be planted in a hole where the root ball is allowed to sink below the surrounding ground level. Therefore it is generally desirable to plant trees into slightly raised beds of well cultivated soil. This improves drainage and provides near surface roots with well aerated soil in which to grow.
Staking may be necessary for some trees which appear unstable and newly planted trees will require watering during dry periods. Tall plants and those in windy positions require staking to stabilize the root ball until established. Position the stakes in the hole before planting and place the plant between them. Long term slow release fertilisers may be added at this stage. As soil is placed in the prepared hole tread firmly to bring soil in close contact with the root-ball. Use wide ties that hold securely without chafing. Tie firmly but allow room for the trunk to increase in girth without constriction. This allows the plant to move a little in the wind encouraging the development of a strong root system without the risk of chafing or root damage. Unless the soil is very wet, water thoroughly making sure that moisture penetrates to the depth of the root-ball.
Mulching annually helps suppress weeds and conserve moisture. Organic materials such as sawdust and bark contribute to soil structure as they decompose but keep mulching material away from the trunk. They benefit from a dressing (50g/m2) of general purpose fertiliser in early spring as new growth begins this will encourage more vigorous healthy growth. Sprinkle evenly and work into the top 2 to 3cm of the soil taking care not to damage surface roots.
The first summer and autumn after planting is critical for young plants; water thoroughly during dry periods. Pests and diseases can have serious debilitating effects on young plants; check regularly. Check stakes and ties for chafing and constriction and remove as soon as plants are strong enough to withstand winds without damage.
Pruning is not normally required for this plant other that to take out any branches that have died or are rubbing.
Ecological and biodiversity benefits
Fruits mature to a dark blue and are attractive to birds. Although flowers are not showy they are an excellent nectar source for bees.
Pests and diseases
No pest and disease issues.
Location at Auckland Botanic Gardens
Interesting facts and tips
Sour gum is a slow-growing deciduous Missouri native tree which occurs in a wide range of soils south of the Missouri River in the southeastern quarter of the State. It is primarily a lowland tree found in low wet woods, bottomlands and pond peripheries, but also can be found on dry rocky wooded slopes and ravines. This tree is primarily dioecious (separate male and female trees) but each tree often has some perfect flowers.