Fertilising guide
How to fertilise Spider Iris (Ferraria crispa)— schedule & NPK
Also called Spider Iris, Starfish Iris, Starfish Lily, Black Flag.
More about spider iris
About Spider Iris
Ferraria crispa · also called Spider Iris, Starfish Iris · flowering
Ferraria crispa is a winter-growing South African corm from the Iridaceae family, producing unusual, deeply fringed, star-shaped flowers in shades of brown, purple, and cream with a distinctive spidery appearance and a strong, carrion-like scent that attracts fly pollinators. In the UK it requires frost-free glasshouse or conservatory conditions, as it cannot survive freezing temperatures; outdoors it is only permanent in the mildest frost-free climates. The most critical care point is to keep the corm completely dry during its summer dormancy, as summer moisture will cause rot. Ferraria crispa belongs to the Iridaceae family; iris plants are listed as toxic to cats and dogs on the ASPCA database, and this genus should be treated accordingly.
Growth habit: Winter-growing, summer-dormant cormous perennial
What fertiliser spider iris actually wants — and why
Spider Iris 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 spider iris: 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 spider iris, 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 spider iris:
Apply a balanced liquid fertiliser monthly during the growing season (autumn through spring); cease feeding entirely during summer dormancy. Treat that as monthly 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 spider iris 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 spider iris
Half strength is the safe default for spider iris — 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 spider iris first if the soil is dry, then apply the diluted feed. The companion question is when to water at all, covered in the spider iris watering schedule.
Signs you are over-feeding spider iris
Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for spider iris:
- 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.
Signs you are under-feeding spider iris
- Uniformly pale or yellow-green leaves, oldest first.
- Noticeably small new leaves and stalled growth in good light and season.
- A generally tired, lacklustre look despite correct watering and light.
If the symptoms point at watering, light or roots rather than nutrition, the full spider iris care brief covers soil, humidity and the common problems for this species.
Flushing and leaching the salts
Flush the pot of spider iris 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 spider iris
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 spider iris — frequently asked questions
What fertiliser does spider iris 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. Spider Iris 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 spider iris?
Apply a balanced liquid fertiliser monthly during the growing season (autumn through spring); cease feeding entirely during summer dormancy. Apply a balanced liquid fertiliser monthly during the growing season (autumn through spring); cease feeding entirely during summer dormancy. Treat that as monthly 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 spider iris?
Half strength is the safe default for spider iris — houseplant feeds are formulated strong, and the diluted dose is gentler on the roots while still ample for foliage.
What does over-feeding spider iris 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 spider iris 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 spider iris?
Flush the pot of spider iris 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.
Keep reading
- Spider Iris care — the full brief (light, soil, humidity, problems, pet safety)
- How often to water spider iris — the watering schedule
- The houseplant fertiliser schedule — feeding through the year
- NPK ratio explained — what the three numbers on the bottle mean
- How to fertilise begonia 'corbeille de feu'
- How to fertilise narrow-leaf coneflower
- How to fertilise red valerian
- All 10153 fertilising guides in the Growli library