Hi David,

Thanks for contacting Ask Extension. Agastache plants (Agastache spp.), also called hyssops or hummingbird mints, have strongly scented foliage and attractive flowers. They're generally tough, easy-to-grow plants that withstand winter cold well. The group includes anise hyssop (Agastache foeniculum), which grows in U.S. Department of Agriculture plant hardiness zones 6 through 10, Korean hyssop (Agastache rugosa), which grows in USDA zones 5 through 9, and "Blue Fortune" hyssop (Agastache "Blue Fortune"), which is a hybrid of these two species that grows in USDA zones 5 through 10. Hyssop (Hyssopus officinalis) is the main generic variety known as common hyssop. This type grows well in U.S. Department of Agriculture plant hardiness zones 3 through 10. Although they're all winter-hardy, giving them extra care as cold weather approaches can help them do well the following spring.

Last fall we had an early freeze followed by a very dry fall and the early part of winter was dry too. In 2020 the Front Range experienced a significant freeze event over the Easter weekend, going from highs in the 50's on April 10 to temperatures in the single digits on April 12, 13,14. These severe freezes and lack of precipitation in the fall and winter months are really hard on plants. These conditions could have contributed to your hyssop plants not making it through the winter.

Did you water the plants in the fall and winter? Since both plants didn’t make it through a winter, I suspect that lack of water during the fall and winter months led to their demise. Even Xeriscape plants need supplemental water during the first couple years after planting and all plants need to be watered during the fall and winter if we don’t have any precipitation.

Here is a handout on fall and winter watering: https://extension.colostate.edu/topic-areas/yard-garden/fall-and-winter-watering-7-211/

When the weather turns cold it’s easy to forget about watering outside plants but this is really important if that plant is to survive and come back next spring.

Keep the plant well-watered from late summer into fall, to allow roots to take up water needed to produce nutrients for storage. Aim for about 1 inch of water weekly, including rain. Don't fertilize the plant late in fall because this stimulates tender new growth that's easily harmed by cold.

If the ground doesn't freeze, continue watering the plant lightly every four to six weeks through fall and winter, because roots can still take up small amounts of water under these conditions. In cold-winter areas, give the plant water whenever a warm spell thaws the ground.

Adding a protective layer of organic mulch during the winter also helps a hyssop plant survive the cold, but if the ground in your area freezes during winter, wait until several frosts have occurred. Mulching too early can keep the soil warm, allowing the plant to start growing again when it should stay dormant and conserve energy for the next spring, and new growth might freeze and jeopardize the plant's survival. If you spread mulch too early, this can also keep moisture in the soil and encourage root rot, which can lessen the plant's ability to grow in spring. When spring does arrive, remove the mulch gradually as you see new growth appear.

I hope you’ll try planting hyssop again and with consistent watering during the fall and winter I think you will have success with this plant. Adding some mulch around the plant in late fall will help too.

Please contact Ask Extension again for any gardening questions you have. 

N.O.