How to Grow Aster

Growth Habit: Asters grow 1 to 6 feet tall and 1 to 4 feet wide depending on the types and variety. The plants are upright and bushy with hairy or smooth leaves and daisy-like flowers.

Staking: Stake or cage tall varieties of asters, such as the New England asters, in midsummer to keep the plant from flopping over in fall.

Watering: Keep young plants well-watered. Water established plants in summer during dry periods. Plants that are drought-stressed may not grow as strongly or flower as well in fall.

Fertilizing: Asters usually don't need much extra fertility if grown on fertile soils. Annually add compost in spring along with a ½ cup of an organic balanced fertilizer.

Mulching: Asters are hardy plants and can survive winters in zone 4 easily. Mulching is necessary only on dry soils to maintain soil moisture levels, to prevent weeds, and to protect those varieties that are marginally hardy in your area. Add a 2 to 3-inch thick layer of shredded bark mulch around the base of the plant in spring. Don't allow the mulch to pile up around the stems or it may cause the crown to rot.

Trimming & Pruning: Asters don't require pruning. However, you can keep taller varieties short and bushy by pinching the tops in early June in the North and late June in the South. Remove the top few inches of growth and the aster plant will respond with more side shoots and a bushier plant. Don't pinch the new growth after early July or you'll be removing flower buds for the fall. The shorter, bushier plant may not require staking or caging.

Deadhead (trim) spent flowers in late fall if you don't want your aster plant to self-sow seeds. Asters will spread by seeds and underground rhizomes. Either cut back the whole plant after flowering, since it will be one of the last flowers to bloom in your garden and you'd be cleaning it up soon anyway. Or deadhead the spent flowers on earlier blooming varieties to allow the foliage to naturally yellow and die.

Dividing & Transplanting: The easiest way to propagate asters is to take a division in spring. This will serve a double purpose of creating new plants to share with friends and reducing the size of an overgrown plant. Asters should be divided every 3 to 4 years so they don't get too large. Spring is also the best time to move an existing aster to a new location. In spring, after the ground thaws and plants start to grow, dig up the aster plant and make 6 to 8-inch diameter divisions with a group of leaves and roots. If you want to create just a few divisions, you can separate out pieces from the mother plant along the edge of the root ball, leaving the main plant intact. Replant the division immediately and keep well-watered.

You can also take cuttings of your favorite aster to propagate. In late spring take 2 to 4-inch long cuttings from the stems of the plant. Remove the bottom leaves, dip the cut end in a rooting hormone powder, and stick the cutting in plastic pots filled with loose potting soil. Place the cuttings in bright area out of direct sun and cover the cuttings and pots with a perforated clear plastic bag. Keep watered when dry and they should root within a few months.

You can also collect and save seeds, but it will take a number of years to grow a sizeable, flowering plant.

Cut back aster plants in fall after the foliage yellows to about 2 inches off the ground. Compost the tops.

Pests/ Disease: The biggest pest of asters is powdery mildew disease. This fungal disease causes the leaves to turn white, then yellow and die from midsummer to fall. If severe, it will defoliate the plant and reduce flowering. The best solution to powdery mildew is to grow resistant aster varieties such as 'Winston Churchill' and 'Blue Lake' New York asters, and 'Bluebird' smooth aster. To prevent powdery mildew from getting started, set plants further apart, thin stems from crowded plants in spring and use preventive sprays. Bacillus subtilis is a bacterial spray that fights fungus and works to control mildew before it takes over. You can also use a baking soda solution of 1 tablespoon of baking soda mixed with one gallon of water with a dash of liquid soap. Again, spray early in the season to prevent powdery mildew from taking hold.