Agastache 'Black Adder' (Black Adder agastache) care

Light

Most houseplants will scorch where agastache 'black adder' thrives. Give it the windowsill you'd otherwise leave empty because everything else burned there. Full sun, 6 or more hours daily, for upright stems and the longest flowering. Shade weakens growth, thins the flower spikes and raises the risk of winter loss. A plant moved abruptly from low light to direct sun bleaches in 48 hours — always acclimatise over a week.

Watering

Aim for when the top 4-5 cm of soil is dry, roughly every 7-14 days once established for agastache 'black adder', but treat that as a starting point rather than a rule. A south-facing summer windowsill will dry the pot twice as fast as a north-facing winter room. Lift the pot; if it feels noticeably lighter than it did wet, water it. Water through the first season to root in, then water sparingly. Drought-tolerant once established; wet winter soil causes crown rot and is the usual cause of failure.

Soil and pot

Agastache 'Black Adder' grows best in well-drained, moderately fertile soil. Light, free-draining loam or gritty soil is ideal; it dislikes heavy, wet clay. Sharp drainage, especially in winter, is more important than fertility. Neutral pH around 6.5-7.5 suits it. A pot with a working drainage hole is non-negotiable for this species — even free-draining mix will turn soggy in a closed planter. If you love the look of a decorative pot without a hole, use it as a cachepot around an inner nursery pot you can lift out to water.

Humidity and temperature

Agastache 'Black Adder' sits happiest at around 30-50% humidity and 15-30°C (59-86°F). Prefers drier air and open, breezy positions. Humid, still or crowded conditions encourage powdery mildew and rot. If you keep the room above 15 year-round and avoid placing the plant near a cold draught, a hot radiator, or an air-conditioning vent, you have already handled the two biggest indoor stressors.

Fertilising

Feed agastache 'black adder' sparingly. Light feeder. A single balanced feed or thin compost mulch in spring is enough. Avoid heavy nitrogen, which produces soft, floppy growth and shortens the plant's life. Skip fertiliser entirely on a stressed, recently-repotted, or actively wilting plant — fertiliser salts make damage worse, not better. Wait for a round of healthy new growth before resuming a feeding rhythm.

Common problems

Below are the issues we see most often on agastache 'black adder' in the Growli community. Each is annotated with the most common cause so you know where to start.

• Winter wet rot — The main cause of loss; cold, wet soil rots the crown. Provide sharp drainage and avoid waterlogged or heavy ground.
• Flopping — Stems lean in shade or over-rich soil. Site in full sun and keep soil lean for sturdy, upright growth.
• Short lifespan — Can be short-lived in cool, damp climates. Take cuttings periodically to keep a supply of replacement plants.
• Powdery mildew — Forms in humid, crowded plantings. Space generously, improve airflow, and water at the base rather than overhead.

Propagation

As a sterile-tending named hybrid, propagate vegetatively to stay true: take softwood or basal cuttings in late spring to early summer, or divide established clumps carefully in spring. Seed will not reproduce the cultivar. Propagation is the cheapest, most satisfying way to expand a collection — and it doubles as insurance against losing a mature plant to an accident. Take a backup cutting once the parent is established and healthy.

Toxicity to pets

Agastache 'Black Adder' is mildly toxic to pets. Agastache 'Black Adder' is a hybrid not individually listed by the ASPCA; treat with caution and verify with a vet. Its parent anise hyssop, Agastache foeniculum, is ASPCA-listed as non-toxic to cats, dogs and horses, but this cultivar is not specifically cleared, and its aromatic oils may cause mild GI upset if eaten in quantity. If you keep cats, dogs, or curious children in the house, weigh placement carefully — a high shelf or a hanging planter is enough for casual safety. For severe ingestion incidents, call your local vet and the ASPCA Animal Poison Control Center (in the US, 888-426-4435).

Pet-safety status is sourced from the ASPCA Toxic and Non-Toxic Plant List, which catalogues the most-asked-about plants for cats, dogs, and horses.

Agastache 'Black Adder' deep-dive guides

Every aspect of agastache 'black adder' care, each with its own calibrated guide:

• Agastache 'Black Adder' watering schedule
• Agastache 'Black Adder' light requirements
• Best soil mix for agastache 'black adder'
• Agastache 'Black Adder' fertilizing guide
• When to repot agastache 'black adder'
• How to propagate agastache 'black adder'
• Agastache 'Black Adder' growth rate & size
• Agastache 'Black Adder' cold hardiness
• Agastache 'Black Adder' temperature & humidity
• Is agastache 'black adder' toxic to cats & dogs?
• Is agastache 'black adder' toxic to cats?
• Is agastache 'black adder' toxic to dogs?
• Getting agastache 'black adder' to bloom

Featured in these plant shortlists

Agastache 'Black Adder' qualifies for 4 curated Growli shortlists — each one filtered objectively from our structured plant-care library, so the selection is consistent and checkable:

• Best drought-tolerant houseplants — Houseplants that prefer to dry out — forgiving of forgotten watering and ideal for travel or busy weeks.
• Best flowering houseplants — Indoor plants grown for their blooms — selected from the flowering species in Growli’s plant-care library.
• Best houseplants for full sun — Houseplants that want direct sun — the species for a hot south or west-facing windowsill where shade-lovers scorch.
• Best fast-growing houseplants — Houseplants documented as fast or vigorous growers — quick to fill a pot, cover a pole or trail down a shelf.
• Browse all 29 plant shortlists — pet-safe, low-light, drought-tolerant and more

Related guides

Agastache 'Black Adder' is also commonly called Black Adder agastache.

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