In short: Coco coir is slightly acidic — buffered coir sits at pH 5.5–6.5 — which is close to neutral for most garden plants and is why it needs no lime, unlike peat. It is not ericaceous. Blueberries, rhododendrons, camellias, azaleas and heathers want pH 4.5–5.5, so plain coir is a fraction too sweet for them. The fix is easy: blend coir with composted conifer bark and pine-needle or leaf mould, feed with an ericaceous fertiliser and water with rainwater, and you have a peat-free ericaceous mix. Coir on its own is fine for acers, hydrangeas and everything else that merely tolerates acidity.
What pH is coco coir?
Raw coco coir lands somewhere between about pH 5.5 and 6.8 depending on where it was grown and how it was washed. Our washed, calcium-buffered bricks are tested to pH 5.5–6.5 batch by batch. For comparison:
| Material | Typical pH | Reading |
|---|---|---|
| Sphagnum peat | 3.5–4.5 | Strongly acidic — peat composts are limed to be usable |
| Ericaceous compost | 4.5–5.5 | Acidic — what acid-lovers need |
| Buffered coco coir | 5.5–6.5 | Slightly acidic — suits most plants without lime |
| Multipurpose compost | 5.5–6.5 | Slightly acidic to near neutral |
| John Innes loam compost | 6.0–7.0 | Near neutral |
| Tap water in hard-water areas | 7.5–8.5 | Alkaline — pushes any pot mix upwards over time |
So "is coco coir acidic?" has two honest answers. Compared with peat, no — it is far milder, which is precisely why coir-based composts need no lime and why coir behaves so predictably. Compared with neutral, slightly, which is where most vegetables, herbs, houseplants and bedding plants are happiest. Does buffering change this? No — buffering loads the coir's exchange sites with calcium so it stops stealing calcium and magnesium from your feed; it is not liming and it does not push the pH up. How to test pH and EC yourself.
Which plants need ericaceous conditions — and which just tolerate them
| Plant | Needs pH 4.5–5.5 (true ericaceous) | Happy in plain buffered coir mixes (pH 5.5–6.5) |
|---|---|---|
| Blueberries, cranberries | Yes — RHS: pH 4.5–5.5, grow in peat-free ericaceous compost if your soil is not acid | No |
| Rhododendrons, azaleas, camellias, pieris, heathers (Erica, Calluna) | Yes | No |
| Japanese acers | No — they prefer neutral to slightly acid and free drainage | Yes |
| Hydrangea macrophylla | Only if you want blue flowers (acid soil plus available aluminium) | Yes — pink or mauve in coir mixes |
| Magnolias, skimmia, some ferns | Prefer acid, tolerate slightly acid | Usually yes |
| Roses, tomatoes, vegetables, herbs, most houseplants | No | Yes |
The RHS rule of thumb for blueberries is a good test: if azaleas, rhododendrons and camellias thrive in your garden soil, blueberries will too; if not, grow them in large pots of peat-free ericaceous compost.
A peat-free ericaceous mix from a coco coir brick
The RHS lists the low-pH organic materials to use around blueberries as pine needles, leaf mould, composted conifer bark and bracken, and warns off farmyard manure and mushroom compost because both raise pH. Build the mix from the same ingredients:
| Ingredient | Parts by volume | Role |
|---|---|---|
| Buffered coco coir, expanded | 5 | Water-holding, air, consistent slightly-acid base |
| Composted conifer (pine) bark, fine grade | 3 | Acidic, long-lasting structure; the backbone of commercial peat-free ericaceous composts |
| Pine-needle mould or leaf mould | 2 | Acidic organic matter and fungal life that ericaceous roots like |
| Optional: sulphur chips | Pack rate | Slowly lowers pH over months — useful for long-term pots, not a quick fix |
- Expand the coir (20 L of water per 5 kg brick) and let it drain to damp.
- Mix with the bark and leaf mould until even. No lime, no manure, no mushroom compost.
- Pot up, water in with rainwater, and test the pH with a cheap kit after a fortnight. Most batches land in the low-to-mid 5s; if yours reads higher, add more bark and pine-needle mould, and give the sulphur time.
- Feed through the growing season with an ericaceous liquid feed, which supplies iron in a form acid-lovers can take up and keeps the mix acidic.
- Water with rainwater. In hard-water areas tap water is the commonest reason an ericaceous pot drifts alkaline and the leaves yellow between the veins (lime-induced chlorosis).
Our ready-blended Ericaceous Mix follows the same logic, adjusted to pH 4.5–5.5 with husk chips for structure — it is not yet on sale, so the recipe above is the way to make it today.
Growing blueberries in coir: the essentials
- Pot size: 30 cm to start, potting on to 45–50 cm after two or three years; blueberries are shallow-rooted and hate drying out, which suits coir's water-holding.
- Mulch the surface with pine bark or pine needles to keep the roots cool and the pH down.
- Two varieties planted near each other crop more heavily than one.
- Re-pot or top-dress with fresh ericaceous mix every couple of years; coir mixes keep their structure well, so a full re-pot is rarely urgent.
Frequently asked questions
Is coco coir acidic or alkaline?
Slightly acidic — pH 5.5–6.5 for buffered coir. Milder than peat (3.5–4.5) and a little below neutral, which is where most plants want to be.
Is coco coir ericaceous?
No. Ericaceous plants want pH 4.5–5.5; plain coir is a little too high. Blend it with composted conifer bark and pine-needle mould, feed with ericaceous fertiliser and use rainwater, and it works well.
Can I grow blueberries in coco coir?
Yes, in an acidified coir-and-bark mix as above, not in coir alone. The RHS target is pH 4.5–5.5.
Do acers need ericaceous compost?
No — they tolerate a range from acid to neutral and care more about drainage and shelter. A coir, compost and grit mix suits them; ericaceous compost does no harm but is not essential.
Does buffered coir raise the pH?
No. Buffering pre-loads the coir's exchange sites with calcium so it does not lock up calcium and magnesium from your feed; the pH stays at 5.5–6.5. It is not liming.
Why do my blueberry leaves turn yellow between the veins?
Usually lime-induced chlorosis — the pH has crept up, often from tap water, so iron is locked out. Switch to rainwater, feed with an ericaceous feed containing chelated iron, and check the pH.
Washed, calcium-buffered coco coir, pH 5.5–6.5 and tested every batch — 75 litres per 5 kg brick, peat-free, free UK delivery.
Shop the 5 kg brick → Mix ratios for every plant type