Trimming & Pruning: Trim back Agastache plants by pinching the new growth in spring to promote bushier growth. Deadhead (trim) spent flower stalks to keep the plant tidy. If growing Agastache as a perennial, don't prune or deadhead past midsummer. Pruning too late in the season will stimulate new growth in fall that may not survive the winter.
If growing Agastache as an annual, keep trimming and deadheading as needed. If you want to prevent Agastache from sowing seeds, which it does rampantly, deadhead before the flower stalks turn brown and the seeds mature and drop to the ground.
In zone 5 or warmer areas where Agastache is a perennial, each spring cut back the plant by 1/3rd to stimulate new growth.
Dividing & Transplanting: Agastache is grown as an annual in cooler climates and a perennial in warmer climates. There's no need for dividing plants in cooler climates. Where Agastache is a perennial it will usually brown and dieback to the ground in winter. For perennials, every 3 to 5 years, dig and divide the plant in spring to rejuvenate it, taking one-foot diameter sections to replant in a location with similar growing conditions.
Pests & Disease: Agastache is relatively pest free. During dry weather spider mites may attack, forming webbing and causing leaves to curl and yellow. Wash off leaves regularly during periods of dry weather to prevent sider mite buildup. Reduce pesticide use so as not to kill predators of spider mites, but if need be, spray insecticidal soap to kill small infestations.
Since Agastache loves well-drained soil, wet, clay soils and areas with lots of winter rains can cause bacteria and fungal growth to rot the plant crown. Prevention is best to stop this rotting. Grow plants on sandy loam soils or build raised beds and bring in the appropriate soil for best growth.