Hyssop (Hyssopus officinalis) is a small, semi-woody perennial herb in the mint family, native to southern Europe and the eastern Mediterranean, where it has a long history as both a culinary seasoning and a medicinal plant. Despite the similar name, it isn't related to anise hyssop; true hyssop belongs to its own genus and carries a sharper, bitter, minty-camphor flavor quite different from anise hyssop's sweet licorice scent. In Kentucky gardens it grows as a fully hardy perennial, forming a low, bushy clump that holds some structure through winter. Hyssop is a straightforward herb to grow, needing little beyond full sun and soil that drains well, though it does best with a hard prune each spring to keep it compact rather than sprawling and woody.
Sowing and planting
Start seed indoors 6 to 8 weeks before the last expected frost, around mid-March in zone 6b, pressing it onto the surface of a moist seed-starting mix since light aids germination. Keep the tray around 65 to 70°F (18 to 21°C); germination is often slow and uneven, taking anywhere from 2 to 4 weeks. Because seed-grown hyssop can vary in flavor and habit from plant to plant, many gardeners prefer to start from stem cuttings or by dividing an established plant, both of which reproduce the parent reliably. Transplant or direct-sow once the danger of frost has passed, spacing plants 18 inches (45 cm) apart in full sun; hyssop tolerates poor, even slightly alkaline soil without complaint, but avoid heavy, wet clay, which this herb handles poorly. Water in well immediately after planting to settle the roots.
Growing on
Water regularly through the first season while plants establish a root system, then ease off; mature hyssop is drought-tolerant and prefers soil on the dry side, while wet soil, particularly through winter, is the most common cause of trouble. Little fertilizer is needed beyond a yearly topdressing of compost, since overly rich soil pushes soft, lanky growth that's both weaker and less aromatic than growth from leaner conditions. The most important routine task is a hard prune in early spring, cutting the plant back by about a third to half and removing old woody stems; skip this and hyssop quickly turns leggy and sparse, with a bare, open center within a few years. A light trim after the first flush of flowers fades can also encourage a second round of bloom later in the season. Weed around young plants, since established hyssop's dense growth shades out most competition on its own.
Harvesting
Leaves can be picked as soon as the plant is established and growing well, generally from about 8 to 10 weeks after sowing. As a perennial shrub rather than a single-harvest crop, it keeps producing new growth to pick all season, right up until several weeks before the first fall frost, which typically arrives in Kentucky by mid-October. Flavor is strongest in leaves picked just before flowering, in the morning once dew has dried and the plant's essential oils are most concentrated. Snip stem tips rather than stripping whole branches, taking no more than about a third of the plant at any one time so it keeps growing strongly. Flower spikes open from mid- to late summer and can be picked individually for fresh use or cut as whole stems for drying; hang bunches in a warm, airy, shaded spot. Dry leaves the same way, then once brittle strip them from the stems and store airtight out of light, where the bitter, minty-camphor flavor holds for several months to a year.
Problems and pests
Leaves yellowing with soft, soggy stems at the base
Root and crown rot set in when hyssop sits in wet, poorly drained soil, especially through a cold winter. Plant in raised beds or mounded rows where drainage is uncertain, and resist the urge to water established plants beyond what rainfall already provides.
Grayish-white powdery coating on the leaves
Powdery mildew turns up most often in late summer, when humid days follow cool nights and plants are crowded with poor airflow between them. Space plants a full 18 inches (45 cm) apart, prune annually to open up the center of the clump, and water at soil level rather than overhead.
Small, ragged holes in young leaves
Flea beetles chew tiny shot-holes in new foliage, particularly on seedlings and freshly planted starts. Established plants generally outgrow the damage without treatment, but a floating row cover (a lightweight fabric laid over the bed) protects vulnerable young plants until they toughen up.
Plant turning woody and sparse in the middle
A hyssop clump left unpruned for several years develops a thin, open center with most of the foliage pushed out to the edges on bare, woody stems. Make the hard spring prune an annual habit rather than an occasional one, and take cuttings to start replacement plants every four or five years, since older plants rarely recover their fullness once badly neglected.
Aphids clustering on new growth
Aphids sometimes gather on tender new shoot tips in spring, especially after a flush of soft growth from overly rich soil. A firm spray of water knocks most of them off, and they rarely do lasting damage to an otherwise healthy plant; ladybugs and other predators usually bring a light infestation under control on their own within a few weeks.
Varieties worth growing
- Blue hyssop (species type) – The common form of the species, with the classic blue-purple flower spikes and the strongest, most traditional bitter-mint flavor.
- Hyssopus officinalis f. albus – A white-flowered form, similar in habit and flavor to the species but useful where a pale color is wanted in the border.
- Hyssopus officinalis f. roseus – A pink-flowered form that adds color variety to an herb garden planted with several hyssops together.
- Rock hyssop (Hyssopus officinalis subsp. aristatus) – A naturally compact, lower-mounding subspecies well suited to edging, rock gardens, or containers.
- Anise hyssop (Agastache foeniculum) – Not a true hyssop despite the name, but a similar- looking, similarly easy perennial herb with a sweeter, licorice flavor better suited to tea.