The Bounty of Giant Hyssop

By Kate Wilson

You step outside your back door and hear the mid-summer hum of insects. As you move in closer you realize it’s the giant hyssop – bees, some wasps and even a couple of butterflies are busy amongst the profusion of compact blue-purple flowers.

Among Edmonton area wildflowers that are ideal for beginners, giant hyssop (Agastache foeniculum) is surely a contender. Its hardiness, relative ease of propagation and striking lavender-blue flower spikes make it not only a joy to grow but a great addition to your garden’s colour palette and pollinator potential.

Your native garden can still offer aesthetic appeal and wildlife habitat through fall and winter. The top of these giant hyssop stalks were partially cut back, leaving stems for overwintering insects and to add texture and contrast against the oncoming snow. (Agastache foeniculum, Alberta Avenue Community League, 2019-09. Photo: K. Wilson.)

A member of the mint (Lamiaceae) family, giant hyssop is also known as blue giant hyssop or anise hyssop. It is not to be confused with Hyssopus officinalis, which is native to the Mediterranean and Middle East.

While giant hyssop is one of our more stand-out native wildflowers – growing at the edge of aspen woodlands or in more open grassy areas – it’s an unlikely discovery around Edmonton. Its small seeds, moderately slow growth and need for sun make it less competitive. But because it shows up where there’s less disturbance, it’s a good indicator the area is somewhat untouched and a special treat if you spot it. 

While giant hyssop’s natural range is from Nebraska to the Northwest Territories, and BC to New Brunswick, it is mostly found in the Great Plains and other grassland habitats such as aspen parkland. I’ve seen giant hyssop as far north as High Level, Alberta. In the Edmonton area, I’ve encountered scattered individuals in the open woodlands along the creek at the corner of Hwy 16 and Hwy 21, east of the city. 

Identifying giant hyssop

In your garden, giant hyssop is dominant in height, growing 60 – 120 cm tall. Stems are 4-sided – characteristic of the mint family – and the crinkly, medium- to dark-green leaves have serrated edges and prominent veins. New leaves may have a purple tint. 

Although its leaf scent has been described as anise, science has found that the chemical associated with anise or licorice is not present in high amounts. Some say the leaves have a scent more akin to basil or French tarragon. 

Agastache foeniculum blooms from mid summer to early fall, with bright blue-purple flowers on a spike-like form. The leaves are opposite on four-sided stems. (Photo: J. Golub.)

In your garden

Giant hyssop fits well into a naturalized garden or mixed native planting, in the back of your bed or as a clumped vertical form within the bed. It’s also a natural choice for your herb garden or pollinator bed. 

In the right conditions, giant hyssop is easy to grow. It tolerates partial shade but prefers full sun; however a critical factor is good soil drainage – excess moisture can cause foliar disease, fungus and root rot, which is fatal.  

Having a taproot system – along with short rhizomes – and evolved to survive in more arid conditions, it is relatively drought tolerant. Flowering is from mid to late June to September, so look forward to a display through to early fall. 

The clump-forming shape will not take over your bed, but in good conditions giant hyssop will spread out from its initial planting – either from an expanding root mass or natural seed propagation. And if you’re looking for a deer resistant plant, its strong aroma and flavour make it resistant to browsing by deer and rabbits. 

Note: Take care if you’re looking for giant hyssop at a greenhouse. Agastache rugosa, a native of east Asia, and A. scrophulariifolia – native to central Canada and northeastern US – are sometimes sold as A. foeniculum.

Propagation

One giant hyssop plant may produce upwards of 90,000 individual flowers. So while you may want to collect seeds, remember that in the right conditions giant hyssop will self-seed. 

These giant hyssop volunteers were potted up after appearing in an adjacent raised bed in Edmonton. Note the purplish colour to some of the new leaves. (Eastwood Community Garden, 2019. Photo: Kate Wilson.)

To collect seeds, wait for early to mid fall when each flower becomes a small capsule, or nutlet. Cut off the seedheads and store in a paper bag, or you can place a bag over the fading flowers to collect the seeds as they dry.

I’ve found the best approach is to let all the seedheads dry out in the paper bag – broken head fragments, seeds and all. After a couple of days of drying, the seeds will have fallen to the bottom. Use a fine sieve or screen to separate the remaining seeds still in the dried debris. At this stage, you’ll find a magnifying glass an invaluable tool for distinguishing the tiny, near-black seeds. You might also consider joining one of the Edmonton Native Plant Society’s seed cleaning sessions. It’s a good way to learn some techniques and get first-hand practice.

While giant hyssop seeds don’t need to be stratified, their germination is improved with at least 6 weeks of cold stratification. Alternatively, sow them in the fall. They need sunlight to germinate, so scatter them on top of the soil. Make sure it’s well drained – a sandy loam or even potting soil is the best option – and where there will be lots of sun and low competition from other plants come spring.

As a local native plant, the best way to ensure you have Edmonton-area progeny is to get seeds or seedlings from a reputable grower. The Edmonton Native Plant Society has both.

Note: There are commercial cultivars of giant hyssop, so if you’re growing native specimens in your bed, be sure to keep other cultivars at a distance or don’t grow them at all, as giant hyssop will happily hybridize with other members of the Agastache family.

Pollinators and wildlife

Considered a basic for a pollinator garden – with its profuse flowers and long blooming season – giant hyssop is so valuable to bees that apiarists consider it a resource in places where it’s being cultivated. The seeds are a food source for birds, and hummingbirds feed on the nectar in its tubular flowers. One garden blogger discovered that various beetles were feeding on the foliage.

In your home

Giant hyssop is edible and is known for its medicinal benefits. If you value essential oils, its oils are rich in antioxidants and have anti-fungal and antimicrobial properties.

Agastache foeniculum, growing in the Nisku Prairie, 2022-07-14.  Photo: P. Cotterill.

Add the leaves as a seasoning to pasta or soups, in jellies or raw in salads. And if you have even a small over-supply of seeds, add them to cookies or muffins. Possibly its most common use is for tea, and the best time to harvest is when the flowers are just past full bloom.

It is also a great addition to potpourri, and if you like to bring nature into your décor, the flower spikes are well suited to fresh or dried flower arrangements.

Giant hyssop is a reliable and versatile plant, one that will reward you year after year. No wonder it was herb of the year in 2019, as chosen by the International Herb Association.