Fertilising guide

How to fertilise Herb Robert (Geranium robertianum)— schedule & NPK

Also called Herb Robert, Robert Geranium, Stinking Bob, Red Robin.

More about herb robert

About Herb Robert

Geranium robertianum · also called Herb Robert, Robert Geranium · flowering

Geranium robertianum is a native annual or biennial wildflower of Europe, North America and western Asia, found in woodlands, hedgerows and shaded rocky ground. It thrives in almost any soil in sun or partial shade and self-seeds so freely that a single plant can colonise a border within a season. The most important care fact is to remove unwanted seedlings early while still small, as the taproot toughens quickly. True Geranium species are not listed as toxic to pets by the ASPCA — this species is considered non-toxic to cats and dogs.

Growth habit: Sprawling, branching annual or biennial forming loose mounds up to 40 cm across, with deeply dissected aromatic leaves and reddish older stems.

What fertiliser herb robert actually wants — and why

Herb Robert flowers best on poor soil — feed it and you get a lush leafy plant with very few blooms, the exact opposite of what you want.

Little or nothing. Rich, especially nitrogen-rich, soil pushes foliage at the expense of flowers in this plant — lean ground is the technique, not a deficiency.

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 herb robert: 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 herb robert, 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 herb robert:

No feeding required — performs best in lean soils; excessive nutrients encourage lush foliage at the expense of flowers. In practice: no routine feeding at all for herb robert — at most a thin compost mulch for soil structure, never a flowering or nitrogen feed.

The dormant-season rule matters more than the exact interval: skip feeding entirely when herb robert 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 herb robert

None is the correct answer for herb robert. The flower-versus-foliage trade-off is the whole point: hold back and you get the display.

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

Signs you are over-feeding herb robert

Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for herb robert:

Abundant leafy growth and very few flowers (the classic over-rich symptom).
Soft, floppy stems and a sprawling, leafy habit.
Scorched edges and salt crust if it has been fed in a container.

Signs you are under-feeding herb robert

Effectively never an issue — these plants flower on poverty.
Only on genuinely dead soil: weak, thin growth and few blooms.
A short-lived plant in completely spent container compost.

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

Flushing and leaching the salts

If herb robert has accidentally been fed and is all leaf, a plain-water flush plus a move to leaner soil resets it; otherwise no flushing is needed because you are not feeding it.

Organic vs synthetic feeds for herb robert

Organic options

A thin compost mulch for soil structure is the absolute most; mostly, give it nothing. UK/US: leave it lean — no manure, no liquid feed. Poor soil is the active ingredient here.

Synthetic / liquid feeds

None. Synthetic feeds, particularly anything with appreciable nitrogen, directly suppress flowering in herb robert.

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 herb robert — frequently asked questions

What fertiliser does herb robert need?

Little or nothing. Rich, especially nitrogen-rich, soil pushes foliage at the expense of flowers in this plant — lean ground is the technique, not a deficiency. Herb Robert flowers best on poor soil — feed it and you get a lush leafy plant with very few blooms, the exact opposite of what you want.

How often should I feed herb robert?

No feeding required — performs best in lean soils; excessive nutrients encourage lush foliage at the expense of flowers. No feeding required — performs best in lean soils; excessive nutrients encourage lush foliage at the expense of flowers. In practice: no routine feeding at all for herb robert — at most a thin compost mulch for soil structure, never a flowering or nitrogen feed.

What strength of feed for herb robert?