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Giant Hyssop, Agastache

Sarah Stehr Sarah StehrSilke Eberhard Silke Eberhard

The ever-growing range of giant hyssop offers a great selection of pretty flower spikes. These are also very popular among butterflies and bees.

Origin

With its cute flower spikes, fresh scent and very long flowering period, giant hyssop (Agastache) has gained more and more fans among garden hobbyists in recent years. This member of the mint family (Lamiaceae) also attracts countless insects with its flowers, and the flowers and leaves of various species can even be harvested and eaten. But things get confusing when it comes to the names of species and varieties in this genus of herbaceous perennials. Some plants are referred to as mints and hummingbird mints while others are called hyssops even though they are only distantly related to true hyssop (Hyssopus officinalis).

Appearance and Growth

How the Giant Hyssop grows depends greatly on which group the relevant species belongs to. Agastache can be categorized into two groups. The four species from China, Korea and North America belong to the first group of garden perennials, along with their varieties, and not only grow significantly taller than the species in the second group but also grow very bushy. Their upright stalks, covered all over with broad, intensely aromatic leaves, form dense clusters. These species can grow to over 8 feet tall. The heat-loving species that make up the second group do not reach these heights and stand out through less densely branched growth. None of them grow taller than 3.3 feet, with some hardly reaching the 12 inch mark.

The species in each agastache group differ considerably in terms of growth and flowers: here are Agastache rugosa (left) and Agastache aurantiaca “Apricot Sprite” (right)

But it’s not just their growth, but also their foliage and flowers that differ. While flowers grow on the four garden perennials in dense pseudo-spikes and are generally white or blue - with the exception of Agastache nepetoides with its yellow-green flowers - the species in the second group show off colorful carpets of pink, red or orange flowers depending on the species and variety. Unlike the rather compact spikes of the first group, they are not only larger but also arranged in groups or whorls, and the individual flowers are less tight. Both groups have the same flowering period though, stretching from late June to September depending on the species and variety. The second group’s leaves are rather linear compared to the first group’s broad, ovate leaves. Depending on the species and variety, the edges of the leaves may be serrated or smooth. All agastache species have characteristic square stems and a striking aniseed or fennel scent given off by the leaves when rubbed.

Location and Soil

In general, giant hyssops are more winter hardy the drier they are. These perennials do not cope well with waterlogging and wet winters. A planting location in full sun with nutrient-rich, permeable soil is ideal. Agastache rugosa and its hybrids tolerate slightly wetter and heavier soils but all others require the addition of sand for increased permeability.

White-green agastache

Some agastache species and varieties even have white-green flowers

Planting

Agastache can be planted into pots from spring to fall. If your garden soil is not very rich in nutrients, you should improve it with some well-rotted compost before planting.

Care Tips

In a sunny location with dry, permeable soil, agastache self-seeds voraciously. And that’s good, as these perennials often don’t live long. If you want to prevent self-seeding, you can cut back the flower heads before the seeds develop. However, this does mean forfeiting their beautiful silhouettes in winter - and taller varieties really add structure to a perennial bed even through the colder months.

Long-lasting giant hyssop species and their hybrids can be divided in spring, both propagating and rejuvenating the plants. Spring is also the time to feed these perennials with compost or another organic fertilizer to make it easier for the plant to grow new growth and begin the new season.

Use

In summer, agastache looks beautiful paired with scarlet bee balm (scarlet monarda), coneflowers (Rudbeckia), globe thistles (Echinops), phlox or scabious. The striking pseudo-spikes of the species used as garden perennials make them predestined for combining with perennials with flowers of other shapes. The compact spikes really pop in combination with species such as goldenrods (Solidago) or white gaura, which have more relaxed inflorescences. Typical fall grasses such as switchgrass (Panicum virgatum) make great bedding partners too. The deep blue-purple of some giant hyssop varieties, such as the Agastache rugosa hybrids “Black Adder” or “Blue Fortune”, create a gorgeous contrast against yellow-flowering shrubs.

Ball dahlia “Jowey Winnie” and purple giant hyssop

Here, orange ball dahlia “Jowey Winnie” contrasts beautifully with blue-purple flowering giant hyssop

Some agastache varieties can also be used in the kitchen. Originating from South-East Asia, purple giant hyssop (Agastache rugosa) has an aroma reminiscent of mint and aniseed. In Korea, it is used to season pancakes and stews. Also known as Korean mint, it can also be used to make a lovely tea. Anise hyssop comes from North America and has an intense aroma of aniseed and fennel that has become its namesake. In the kitchen, it can be used in place of these herbs, such as in a fish stew or salad. Agastache mexicana is native to Mexico and its citrus flavor is ideal in refreshing drinks.

Important Species and Varieties

As already mentioned, the giant hyssop genus includes around 22 perennial species that can be split up into two very different groups. On one hand, there are species from China, Korea and North America such as Korean mint or Agastache rugosa, anise hyssop (Agastache foeniculum), Agastache nepetoides and horse mint (Agastache urtifolia), which are all grown as classic garden perennials in temperate zones. Various varieties are available to buy, such as Agastache rugosa hybrids “Black Adder” and “Blue Fortune”.

The second group includes species from warmer climes such as Mexican giant hyssop (Agastache mexicana) or Agastache aurantiaca. They naturally grow in open, dry and stony locations in Mexico and California, and are predominantly grown as annuals in temperate zones due to their sensitivity to frost.

We can thank the giant hyssop’s willingness to cross breed for the fact that the variety of plants available is continuing to grow. For example, there are now varieties such as Agastache mexicana “Sangria” that are more resilient to frost than their wild counterparts.

Agastache mexicana “Sangria”

Agastache mexicana “Sangria” is somewhat frost-sensitive but has good chances of surviving winters outdoors in mild wine-growing climates

Propagation

You can propagate longer-living species of giant hyssop through division in spring. Another possible propagation method is the use of cuttings taken in late summer after flowering. These must be overwintered in a place free from frost after they begin growing. As mentioned, giant hyssop often creates lots of seeds that you can use to sow yourself. These propagation methods only work for these species. Cultivated varieties do not grow true to the variety so must be propagated through division or cuttings.

Diseases and Pests

Overall, agastache is robust and resilient to disease. If the weather is consistently damp, powdery mildew can occur and slugs may be an issue.

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