Hydrangeas & ericaceous

Hydrangea feed and ericaceous plant food: start with the soil

Hydrangeas, rhododendrons, azaleas, camellias, pieris and heathers share one demand that ordinary garden feeds ignore: they want the soil on the acid side. On chalky or limy ground their roots cannot take up iron and manganese, the young leaves go yellow between the veins, and no amount of general fertiliser puts it right. So a hydrangea feed or an ericaceous plant food is as much about the soil as the plant. This collection has the three things that matter: something to lower the pH, something to correct the magnesium that so often runs short alongside, and a balanced shrub feed that gives flower without pushing soft leafy growth.

What to feed hydrangeas

Less than most people think, and later than most people think. A hydrangea in reasonable soil wants one feed as growth starts in spring and, at most, a second in early summer; feeding into late summer produces soft shoots that the first frost takes. The feed itself should be balanced or leaning to potassium rather than nitrogen-heavy, which is why a lawn feed or a general high-nitrogen fertiliser gives you a big green plant with few flowers. Organic Rose & Shrub Fertiliser 5-3-5 is the one to reach for: slow release, balanced, with calcium and magnesium alongside the three main nutrients. For a hydrangea that is meant to be blue there is one further rule, covered below: keep phosphorus low, because it ties up the aluminium the colour depends on.

If the plant is in a pot, feeding matters more and the margin for error is smaller. Containers hold little in reserve, ericaceous compost is low in nutrients to begin with, and frequent watering leaches what is there. Feed little and often through spring and early summer, use rainwater where you can (tap water in hard-water areas is alkaline and slowly undoes the compost), and top-dress rather than repot every year.

Ericaceous plant food: what it needs to be

The label "ericaceous" on a feed means two things, and it is worth checking a product for both. First, it must not raise soil pH: no lime, no calcium carbonate, and nothing that leaves an alkaline residue. Second, it should carry the nutrients acid-loving plants run short of, chiefly magnesium and iron, in forms that stay available at low pH. Everything in this collection passes the first test. Kieserite and Polyhalite are neutral mineral sulphates and do not move pH; Sulphur Soil Acidifier lowers it; and the Rose & Shrub 5-3-5 supplies the balanced feed. An azalea fertiliser or a rhododendron feed is the same specification under a different name: the plants are close relatives and want the same things.

Lowering soil pH: sulphur

Sulphur Soil Acidifier is elemental sulphur in bentonite pastilles. Soil bacteria convert it slowly to sulphuric acid, which is why it works over months rather than days and why it is the safest way to bring a bed down towards the pH 4.5 to 5.5 that rhododendrons and blueberries prefer. How to increase or decrease your soil pH explains how much change to expect and how to measure it. A soil pH test before and after is worth far more than guessing: on a strongly alkaline garden the honest advice is often a large container of ericaceous compost rather than a long campaign against the ground.

Yellow leaves between the veins: magnesium

Interveinal yellowing on the older leaves of a rhododendron or hydrangea is usually magnesium, particularly on light acid soils and after heavy potassium feeding. Kieserite supplies magnesium as a mineral sulphate, and Liquid Magnesium is the faster option through a watering can. If the yellowing is on the young leaves instead, the problem is more likely to be iron locked up by high pH, and the answer is the sulphur above rather than more feed.

Feeding camellias and rhododendrons

Camellias and rhododendrons are shallow-rooted, slow to respond, and commonly short of magnesium, which shows as older leaves yellowing from the edges inwards while the veins stay green. They resent salt, so little and often beats one heavy dose, and they resent disturbance, so feed onto the surface and mulch rather than forking anything in. Polyhalite suits them well: potassium, calcium, magnesium and sulphur in one slow-release granule that does not raise pH. The calcium in it is the one thing to weigh up. A camellia or rhododendron on the borderline of tolerance will not thank you for more of it, so on ground that is already near neutral, use kieserite for the magnesium instead. Seaweed Extract Powder and Fulvic Acid Powder are the biostimulant options, the second of them useful because fulvic acid keeps iron and other trace elements in a form the plant can take up when the soil chemistry is working against it.

Blue hydrangeas

Flower colour in the mophead and lacecap hydrangeas follows aluminium, not any feed as such. Aluminium becomes available to the plant only once the soil is acid enough, roughly pH 5.5 or below, so on acid ground the same plant is blue and on chalk it is pink. Two things follow. Sulphur is the tool, applied over a season or two with a pH test to see where you are. And phosphorus is the enemy: it binds aluminium in the soil, which is why a high-phosphorus bloom feed will hold a hydrangea pink however acid the ground and why there is no bloom booster in this collection. White hydrangeas do not change colour whatever you do. Blueberries want the same acid soil and the same low-phosphorus feeding; they are covered with the rest of the fruit in Fruit Trees & Soft Fruit.

Common questions