How to Grow Hyssop in Australia

Hyssop (Hyssopus officinalis, family Lamiaceae) is a compact, semi-woody perennial grown for its aromatic leaves and blue to violet summer flowers, which bees visit. The store sells plain hyssop seeds, a plant of 40 to 60 cm, and pink and white forms of the same species also exist. It is not anise hyssop, a different plant with its own guide, how to grow anise hyssop. Hyssop shrugs off drought and frost, but every source asks for free-draining soil, because it rots in wet ground.

Three mistakes that matter

1. Wet, heavy or rich soil. Hyssop is a Mediterranean herb that likes light, rather dry soil, and roots left in cold, wet clay rot. Plant it in light, free-draining soil or a raised bed, and water only once the top few centimetres dry out.
2. Burying the seed. Seed covered deeply often fails to show. Sow it no deeper than 3 mm, keep the tray in bright light, and see Germinating the seed.
3. Never cutting it back. Left alone, the woody base splays open and the plant goes bare in the middle. Shear it after flowering and trim it again in early spring, as Trimming and Cutting Back explains.

When to plant

Seed germinates best with the mix between 18 and 22°C, and it slows once the mix falls below about 15°C. That suits spring in most zones and autumn where winters are mild. An Australian herb grower sows it in spring or autumn, so one sowing each year is plenty for a perennial.

At the cold edge of a window, start seed indoors on a heat mat or a warm, bright sill. In the humid north, sow as the dry season arrives, because summer humidity is what finishes hyssop.

Site, soil and setup

Hyssop grows best where lavender, thyme and rosemary thrive: an open, sunny spot and soil that drains fast after rain.

Container growing

Hyssop suits pots, window boxes and a sunny courtyard, and a pot gives it the sharp drainage it wants. Give one plant a pot of at least 15 L and 30 cm across, filled with a premium potting mix. Stand the pot on feet so water runs straight out.

• Watering: water when the top few centimetres of mix are dry, then let it drain. Never leave the pot in a saucer of water.
• Feeding: a fish and seaweed blend at half strength, about 12 mL per 9 L, fortnightly from spring to late summer.
• Position: full sun, with some afternoon shade in hot districts, where pots dry out fast.
• Renewal: pot on or replace the plant every few years, as older plants go woody and thin.
• Mix safety: potting mix can carry Legionella bacteria, so wet it down, wear gloves and a P2 mask, and wash your hands afterwards.

Germinating the seed

Hyssop seed has no dormancy to break, and it needs no soaking, chilling or scratching. Warmth, steady moisture and a light covering are enough. In seed bank tests at the Royal Botanic Gardens, Kew, untreated seed gave 62 to 100 per cent at 15 to 20°C in daily light.

One Kew batch reached 100 per cent within 14 days at 20°C, while another took 56 days at 15°C to reach 86 per cent. In a tray at 18 to 22°C, seedlings usually show in 14 to 21 days. No germination rate is recorded for home trays, so sow two or three seeds for each plant you want.

For more on which seeds need help and which do not, see soaking, scarification and cold stratification.

When nothing comes up

• Too cold. Below about 15°C, seed sits for weeks. Move the tray to a heat mat or a warmer room.
• Too hot. Mix at 30°C slows germination too. Shade the tray from hot afternoon sun.
• Buried too deep. Sow again with no more than 3 mm of cover.
• Dried out, or soggy. Keep the mix evenly moist but never waterlogged; cover the tray with clear plastic until seedlings show.
• Old seed. Kept in a warm room, viability halves in about three years. Store seed cool and dry in a sealed jar.
• Given up too soon. In cool mix, wait eight weeks before sowing again.

Sowing Seeds

Start hyssop in punnets or cell trays for the most even plants, or sow it where it is to grow once the soil is warm. Both begin from untreated seed, sown shallow, as Germinating the seed explains.

Starting in trays

• Fill a cell tray with fresh seed raising mix, firm it lightly and water it well.
• Sow into each cell at the seed count and depth Germinating the seed gives.
• Keep the tray warm and bright, and water from below.
• Thin each cell to the strongest seedling once the first true leaves appear.
• Pot on into small pots when seedlings are big enough to handle.

Sowing direct

Rake the bed fine, water it, then sow a pinch of seed every 30 to 40 cm and cover it lightly. Keep the surface moist until seedlings show, and thin each spot to one strong plant.

Planting out

• Plant out when seedlings are several centimetres tall and well rooted.
• Harden off first (stand the pots outdoors a little longer each day for a week, so they adjust to sun and wind).
• Plant at the same depth as in the pot, 30 to 40 cm apart, and water in with seaweed solution at half strength.
• Water regularly until the plants settle, then ease off.

More on the move from pot to garden is in hardening off seedlings before planting out.

Care by growth stage

Drainage and restraint decide how well hyssop grows. Over-feeding reduces the essential oils that give hyssop its aroma, so feed lightly, at half strength at most.

If growth stalls in poor soil, a light application of all-purpose controlled release granular (NPK around 14-5-10) scratched in each spring can help.

Trimming and Cutting Back

Two light cuts a year keep hyssop dense: one after flowering and one in early spring.

• Shear off spent flower spikes as they fade, taking a little of the leafy growth with them.
• Trim the whole bush by about a third in early to mid-spring, just as new shoots start.
• Cut into leafy growth, not bare old wood, which is slow to sprout again.
• Shape a low hedge at the same two times, with shears held level.
• Leave frost-damaged tips until spring, then cut back to healthy buds.

For raising new plants from the trimmings, see how to take cuttings.

Companion planting

Choose neighbours that want the same sun, dry soil and lean feeding.

Grows well with

• Catmint, the partner an old English herbal names for a hyssop border.
• Lavender, which likes the same sun and fast drainage.
• Rosemary, another dry-soil shrub that makes a taller backdrop.
• Thyme, a low edging that shares its needs.

Keep apart

• Mint, whose spreading roots and liking for moist soil do not suit a dry bed.
• Thirsty leafy vegies, whose frequent watering keeps hyssop's roots too wet.

Harvesting Leaves and Flowers

Cut flowering tops just as the first flowers open, usually in midsummer. Pick leafy shoots any time the plant is growing strongly.

• Cut in the morning once dew has dried.
• Take no more than a third of a plant at one picking.
• Cut stems with secateurs just above a pair of leaves.
• Tie small bunches and hang them upside down in a dry, airy, shaded room.
• Dry until leaves crumble and stems snap cleanly.

Store dried leaves and flowers in a sealed jar in a cool, dry, dark cupboard, labelled with the botanical name and the picking date. Cut spikes also last in a vase. The leaves and flowering tops carry an essential oil rich in pinocamphone and isopinocamphone, so read Safety before any other use.

Traditional use and history

The name comes from the Greek hyssopos, which Mrs Grieve's A Modern Herbal traces to a holy herb used for cleaning sacred places. The species name officinalis means "of the offices", marking a plant once sold by apothecaries.

The hyssop of the Bible is probably another plant. Grieve notes that tradition links it with a marjoram. The Missouri Botanical Garden names Syrian oregano (Origanum syriacum) as the plant believed to be the Old Testament hyssop. Grieve adds that the garden hyssop does grow in Palestine, but is not conspicuous there.

Hyssop is native to southern and eastern Europe, North Africa and western Asia. Grieve records it naturalised on the ruins of Beaulieu Abbey in England, a sign of old monastery gardens. Kew's Plants of the World Online now files the species as Dracocephalum officinale, while the seed trade still sells it as Hyssopus officinalis.

Safety

Happy Valley Seeds sells hyssop as seed for growing. Nothing here is advice to eat or take any part of the plant. Anyone considering using a plant should first seek advice from a qualified health practitioner.

If someone collapses, stops breathing, has a seizure or has a severe allergic reaction, call triple zero (000). For any suspected poisoning, call the Poisons Information Centre on 13 11 26 without waiting for symptoms.

Common problems

No seedlings. Usually cold mix or seed buried too deep; see Germinating the seed and its list, When nothing comes up.

Tall, pale, floppy seedlings. Too little light on a windowsill stretches them. Move trays to brighter light and turn them daily. Read more

Plants that rot in summer. High summer humidity is the usual cause; in the north, time sowings as When to plant explains, and give the sharpest drainage you can.

Plants that die over a wet winter. Heavy soil that stays wet rots the roots; see Three mistakes that matter.

A bare, woody middle. An unpruned plant splays open with age; see Trimming and Cutting Back.

Leaves dropping in a cold winter. Older plants often lose many leaves in hard cold. They usually reshoot in spring after a trim.

Pests and diseases

Hyssop has few serious pests and is generally free of disease.

Leafhoppers (family Cicadellidae). Overseas records list hyssop as prone to these small, wedge-shaped sap-suckers, which Australian gardens also host. They jump or scuttle sideways when disturbed, leaving pale specks on the leaves. Most gardens need no control; spiders, lacewings and ladybirds eat them.

Root-knot nematode (Meloidogyne species). Nematodes can trouble hyssop, and root-knot nematodes, very common in Queensland soils, attack a wide range of plants. Plants stunt, yellow and wilt in patches, and lifted roots carry small lumps. Lift affected plants, replant in a fresh spot, and dig in compost, which can lower nematode numbers. Read more

Common garden snail (Cornu aspersum) and grey field slug (Deroceras reticulatum). Both graze seedlings at night in damp weather. Hand-pick them after dark, or scatter an iron-based snail bait thinly as the label directs. Read more

Damping off (Pythium species and Rhizoctonia solani). Seedlings rot at soil level and collapse, most often in wet mix. Use fresh seed raising mix and clean trays. Read more

Prevention first

• Keep seed trays clear of rain and hose splash, which spread damping off.
• Water the soil, not the leaves, and avoid overwatering in cool, wet weather.
• Check leaf undersides for leafhoppers weekly through spring and summer.
• Clear leaf litter and loose pots from around seedlings, where snails shelter.

Tips

1. Grow a low hedge of hyssop along a path or the edge of the vegie patch, where bees and butterflies work the flowers.
2. Take tip cuttings in spring or autumn to keep young plants coming on, since hyssop is short-lived.
3. Save seed from a few spikes once they turn brown and dry, and store it in a paper envelope in a cool, dry place.