Hydrangea Fertiliser and Soil pH: How to Feed Them and Change the Colour

Hydrangea fertiliser and soil pH are one decision, not two. The colour of a bigleaf hydrangea is set by aluminium availability, which soil pH controls, while vigour and flower count are set by the fertiliser. So choosing a feed without a pH target is how people end up with a large green plant and no blue. This guide covers what to feed hydrangeas, when to feed them, which NPK and sulphur or aluminium-sulphate rates actually change flower colour, what hydrangea food does and does not do, and the three products that consistently make things worse.

If you already know the colour mechanics and only want the feed rates, jump to the feeding schedule.

What hydrangea fertiliser can and cannot change

Fertiliser controls vigour, leaf colour and the number of buds. It does not turn a hydrangea blue on its own, and no brand of “hydrangea food” contains the ingredient that does. Colour depends on whether aluminium ions stay soluble in the soil solution and get taken up by the roots, and aluminium only stays soluble below about pH 5.5. Above pH 6.0 it locks up in the soil and every flower opens pink, whatever you feed.

So the order of operations is: test the pH, set the target, then choose a fertiliser that does not fight the target.

Goal	Soil pH target	Colour lever	Fertiliser role
Blue flowers	5.2 – 5.5	Aluminium available; aluminium sulfate or elemental sulphur added	Supports growth only
Pink flowers	6.0 – 6.5	Aluminium locked out	Supports growth only
Best blooms either way	5.5 – 6.2, species-dependent	Panicle and oakleaf types stay white or cream regardless	Balanced feeding at moderate strength

Mountain hydrangeas and oakleaf hydrangeas do not change colour with pH at all, their pigments are structural. If yours is one of those, all of the pH work below is optional.

Which NPK hydrangeas want

Hydrangeas are moderate feeders with a preference for balanced to slightly nitrogen-heavy nutrition, and they are sensitive to salt. A balanced 10-10-10 at half strength is the safe default; many growers work to a ratio nearer 4-3-2 (12-9-6 or 8-8-8) because excess phosphorus does nothing useful here and can lock up micronutrients.

• Nitrogen: drives leaf and cane growth. Too much produces soft growth that wilts and flowers late.
• Phosphorus: supports flowering and roots, but most garden soils already have plenty. Extra phosphorus at high pH binds iron and zinc and can induce the very chlorosis people then try to fix with more feed.
• Potassium: winter hardiness, water regulation and colour intensity.

Avoid high-nitrogen lawn fertiliser near hydrangeas, and never apply lime unless a soil test says the pH is genuinely too low, lime raises pH and turns blue hydrangeas pink.

Feeding schedule for hydrangeas

1. Early spring, as buds swell: first application: granular balanced fertiliser, or a half-strength liquid feed. Scratch into the surface and water in.
2. After the first flush of blooms: second application at the same rate, feeding the second flush many mophead varieties produce.
3. Midsummer: only if the plant is vigorous and the leaves are pale; otherwise stop. Late feeding pushes soft growth that winter kills back.
4. Autumn: no fertiliser. Water deeply and mulch instead.

Liquid feeds (a balanced liquid fertiliser at half the label rate, every two weeks through the growing season) suit container plants, where nutrients leach out with every watering. Slow-release granules suit borders, where a single spring application holds the plant to midsummer. Organic options that work well: composted manure in a ring around the drip line, blood meal for a nitrogen kick (acidifying as it breaks down, which suits a blue target), bone meal for phosphorus, and kelp meal for trace elements.

Products people use, and what they actually do

Product	What it does	When it is the right choice
Aluminium sulfate	Supplies soluble aluminium and acidifies, the fastest route to blue	Spring, when a colour change is the goal; small doses, then re-test
Elemental sulphur	Soil bacteria convert it to acid over 4–8 weeks; no aluminium of its own	Controlled pH correction in spring or autumn
Hydrangea food / acidifying fertiliser	Balanced nutrition, usually with sulphur or iron included	Routine feeding when pH is already in range
Epsom salts	Magnesium only. Does not affect colour or acidity	Only if a tissue test shows magnesium deficiency
Vinegar or citric acid	Transient pH swing, no lasting change, harms soil life	Never. Use sulphur instead
Garden lime	Raises pH	Only when a test says pH is too low and you want pink

How to change hydrangea colour: the reliable sequence

1. Test the soil at the drip line, 10 cm deep, in early spring.
2. Pick one target and hold it for a full season. Swinging between treatments produces mixed-colour plants.
3. For blue: aluminium sulfate at label rate in spring (typically twice, four weeks apart), or elemental sulphur worked into the root zone the previous autumn. Hold pH at 5.2–5.5.
4. For pink: stop acidifying entirely; if pH is below 5.8, apply dolomitic lime in autumn and re-test before spring.
5. Wait a full flowering cycle before judging the result. Existing buds are already committed; the response shows on the next flush.

The mechanics behind that (why aluminium and not pigment, why the change lags, what happens at the extremes) are in the existing guide to changing hydrangea colour. The amendment side (rates, safety, and what aluminium sulfate does to soil structure over time) is in the aluminum sulfate guide, and sulphur options are compared in elemental sulphur versus sulphate sulphur.

Soil, watering and placement: the half that gets skipped

• Texture first. Hydrangeas want a moisture-retentive but free-draining loam rich in organic matter. Heavy clay plus high pH is the worst combination for blue, because both reduce aluminium availability.
• Water deeply, less often. Shallow watering gives shallow roots and midday wilt. Two thorough soakings a week beat daily sprinkling.
• Mulch 5–8 cm with compost or bark. It holds moisture, feeds the soil as it breaks down, and slowly acidifies as organic acids are released.
• Afternoon shade stops blooms bleaching, but the plant needs morning light to bud up.

Feeding a hydrangea that is already stressed

Pale yellow leaves with green veins are iron chlorosis, and on hydrangeas that is nearly always a pH problem rather than an iron shortage, the iron is in the soil but unavailable. Add acid, not iron: iron chelate gives a quick green-up, but the correction only holds once pH is back in range. Wilting with black-edged leaves is fertiliser burn; flush the soil with several volumes of water and skip the next feed. No flowers is almost always nitrogen excess, shade, or pruning at the wrong time, not a lack of fertiliser.

Bottom line

Test the pH first, choose one colour target, and let the fertiliser do only its own job: balanced nutrition in spring and after the first flush, no late nitrogen, no lime, no vinegar. If you want blue, the amendment (aluminium sulfate for speed or elemental sulphur for control) does the colour work while the feed keeps the plant strong enough to show it. Re-test at the end of the season, because the pH you set in spring will have moved.

Before you commit a bed to an acid-loving planting scheme, check what else in it will tolerate pH 5.2: the plant pH database filters by exact pH range, sunlight and traits.

Sources

• How Are Hydrangea Flower Colors Determined? (FS309E) — Washington State University Extension; backs Colour depends on whether aluminium ions stay soluble in the soil solution and get taken up by the roots, and aluminium only stays soluble below about pH 5.5..
• Hydrangeas — Alabama Cooperative Extension System; backs Above pH 6.0 it locks up in the soil and every flower opens pink, whatever you feed..
• Acidifying Soil for Blueberries and Ornamental Plants in the Yard and Garden — Oregon State University Extension; backs Use sulphur instead Garden lime Raises pH Only when a test says pH is too low and you want pink How to change hydrangea colour: the reliable sequence Test the soil at the drip line, 10 cm deep, in early spring..
• Growing Tips: Hydrangeas, Color and Fertilizing — University of Massachusetts Amherst Extension; backs A balanced 10-10-10 at half strength is the safe default; many growers work to a ratio nearer 4-3-2 (12-9-6 or 8-8-8) because excess phosphorus does nothing useful here and can lock up micronutrients..