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Fertilising guide

How to fertilise Agastache 'Black Adder' (Agastache 'Black Adder')— schedule & NPK

Also called Black Adder agastache.

More about agastache 'black adder'

About Agastache 'Black Adder'

Agastache 'Black Adder' · also called Black Adder agastache · flowering

Agastache 'Black Adder' is an aromatic hybrid hyssop with dense, smoky violet-blue flower spikes emerging from near-black buds from midsummer to autumn. Vigorous and long-blooming, it wants full sun and free-draining soil, tolerates heat and drought once established, and is a prolific nectar source for bees, butterflies and hummingbirds.

Growth habit: Upright, bushy, clump-forming aromatic perennial (a foeniculum-type hybrid) with mint-family foliage and dense, branching flower spikes over a very long season.

What fertiliser agastache 'black adder' actually wants — and why

Agastache 'Black Adder' is an easy, light foliage feeder — a half-strength balanced liquid feed through the growing months keeps it green without forcing weak, sappy growth.

A balanced general houseplant feed (roughly even N-P-K) is exactly right — it is grown for foliage, so steady, moderate nitrogen for healthy leaves is the goal, not a bloom or root formula.

For the language behind the three numbers on the bottle — what nitrogen, phosphorus and potassium each do — see the NPK ratio explained entry. The short version for agastache 'black adder': match the feed to the job the plant is doing right now, not to a generic “plant food” on the shelf.

How often to feed agastache 'black adder', and which months

Feeding only earns its keep while the plant is in active growth and can use the nutrients — pour feed into a dormant or low-light plant and it simply builds up as root-burning salt. For agastache 'black adder':

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. Treat that as sparingly through the growing season between spring through early autumn (roughly March to September); ease off in autumn and stop entirely in the low light of winter.

The dormant-season rule matters more than the exact interval: skip feeding entirely when agastache 'black adder' is resting. For the wider context on indoor feeding rhythms across the seasons, the houseplant fertiliser schedule walks through the year month by month.

What strength to mix for agastache 'black adder'

Half strength is the safe default for agastache 'black adder' — houseplant feeds are formulated strong, and the diluted dose is gentler on the roots while still ample for foliage.

Feeding always goes onto already-damp soil, never dry roots — water agastache 'black adder' first if the soil is dry, then apply the diluted feed. The companion question is when to water at all, covered in the agastache 'black adder' watering schedule.

Signs you are over-feeding agastache 'black adder'

Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for agastache 'black adder':

Signs you are under-feeding agastache 'black adder'

If the symptoms point at watering, light or roots rather than nutrition, the full agastache 'black adder' care brief covers soil, humidity and the common problems for this species.

Flushing and leaching the salts

Flush the pot of agastache 'black adder' with plain water until it runs freely from the base every couple of months in the feeding season — it washes out the fertiliser salts that cause brown tips.

Organic vs synthetic feeds for agastache 'black adder'

Organic options

A diluted seaweed or worm-casting feed, or fish emulsion if you can tolerate the smell indoors. UK: Westland or Baby Bio Organic, dilute seaweed; US: Espoma Indoor! or Neptune's Harvest fish & seaweed. Slow, gentle and hard to overdo.

Synthetic / liquid feeds

A general-purpose houseplant liquid at half strength — UK: Baby Bio, Westland Houseplant Feed or Phostrogen; US: Miracle-Gro Indoor Plant Food or Schultz. Convenient and fast-acting; the only risk is overdoing it.

Brand names are examples, not endorsements, and UK and US ranges differ — check the label’s own NPK and dilution rate, since formulations change.

Fertilising agastache 'black adder' — frequently asked questions

What fertiliser does agastache 'black adder' need?

A balanced general houseplant feed (roughly even N-P-K) is exactly right — it is grown for foliage, so steady, moderate nitrogen for healthy leaves is the goal, not a bloom or root formula. Agastache 'Black Adder' is an easy, light foliage feeder — a half-strength balanced liquid feed through the growing months keeps it green without forcing weak, sappy growth.

How often should I feed agastache 'black adder'?

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. 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. Treat that as sparingly through the growing season between spring through early autumn (roughly March to September); ease off in autumn and stop entirely in the low light of winter.

What strength of feed for agastache 'black adder'?

Half strength is the safe default for agastache 'black adder' — houseplant feeds are formulated strong, and the diluted dose is gentler on the roots while still ample for foliage.

What does over-feeding agastache 'black adder' look like?

Brown, crispy leaf tips and edges with no sign of underwatering. A white, crusty salt deposit on the soil surface or pot rim. Weak, pale, stretched new growth that flops. Lower leaves yellow and drop while the soil is correctly watered. Feeding agastache 'black adder' year-round on a fixed schedule, including dark winter months, is the most common mistake — it cannot use the nutrients in low light and the surplus simply burns the roots and crusts the soil.

Should I flush the soil of agastache 'black adder'?

Flush the pot of agastache 'black adder' with plain water until it runs freely from the base every couple of months in the feeding season — it washes out the fertiliser salts that cause brown tips.

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