How do I grow blueberries and rhododendrons if my soil is not acid?

Ericaceous plants — blueberries, rhododendrons, azaleas, camellias, pieris, most heathers — evolved on acid soils where iron and manganese are freely soluble. Put them somewhere alkaline and they cannot dissolve the iron that is already in the soil, so the new leaves come through bright yellow with sharp green veins and the plant slowly starves in the middle of plenty.

What you'll need

• Ericaceous compost
• A container of reasonable size, 40 litres or more for a blueberry
• A water butt or another rainwater source
• Ericaceous liquid feed
• Composted bark or pine needles for top-dressing (optional)
• Sulphur chips (optional)
• Chelated iron (optional)

Step by step

1. Establish whether your ground is genuinely hopeless.Test the pH, then dig down 300mm and look. Chalk or limestone in the subsoil means the buffering never stops and open-ground acidification is a permanent losing battle. Neutral soil over clay or sandstone can be shifted, and is worth attempting.
2. Choose a container that is large enough.A blueberry needs at least 40 to 45 litres to crop properly, a rhododendron similar. Small pots dry out, and drought is the second killer of these plants after pH. Terracotta dries faster than plastic or glazed; choose accordingly.
3. Fill with ericaceous compost, not multipurpose with something added.Ericaceous compost is formulated at around pH 4.5 to 5.5 and stays there for a season or two. Adding pine needles or coffee grounds to ordinary compost does not meaningfully acidify it — that is a persistent myth.
4. Set up a rainwater supply before you plant.This is the step people skip and it is the one that decides whether the plant is still healthy in year three. Hard tap water carries dissolved calcium carbonate, and using it steadily raises the pH of the pot. A single butt off a shed roof is enough for a few pots.
5. Plant at the right depth and mulch.Blueberries and rhododendrons are shallow-rooted, so plant with the top of the rootball level with the compost surface and never bury the stem. A 50mm mulch of composted bark or pine needles keeps the surface cool, moist and slightly acid.
6. Feed with an ericaceous feed during the growing season.General fertiliser is usually alkaline-leaning and contains no chelated iron, and it makes the problem worse over time. Feed from spring to midsummer only, and stop by the end of August.
7. Top-dress every spring and repot every few years.Scrape off the top 50mm of compost and replace it with fresh ericaceous. Every three to four years, repot completely into fresh compost — the mix acidifies less and less as it ages.
8. If you must use tap water, buy back the pH.In a drought where the butt is empty, tap water is better than a dead plant. Compensate afterwards with an ericaceous feed and a fresh top-dressing, and get the butt refilled.
9. Consider sulphur only where the subsoil allows it.On neutral, non-chalky ground, elemental sulphur chips applied at the pack rate the autumn before planting will lower a bed by up to a point over a year, as soil bacteria oxidise them. It needs warmth and moisture to work, it takes months, and it needs repeating. It does not work over chalk.

Tips

• Chelated iron gives a rapid green-up on a chlorotic plant and buys you a season while you fix the real problem. It is a rescue, not a regime — it lasts weeks and costs more than moving the plant into a pot.
• Blueberries need two different varieties nearby for a decent crop. Two large pots side by side is a better plan than one big one.
• A raised bed filled with ericaceous compost on top of chalky ground works for a few years and then gradually fails as calcium moves up from below. A container with a solid base does not have this problem.
• Camellias and rhododendrons set next year's flower buds in mid to late summer. Drought at that point costs you the following spring's flowers even though the plant looks fine at the time.

If it doesn't work

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