| On occasion, I have gardens with complex requirements. One of these complexities is working with ericaceous plants, which thrive in acidic soil (pH below 7) and often prefer a pH between 4.5 and 6.5. You would have read my Plot 17 posts, where I talk about how we need to keep our blueberry bushes healthy by keeping the acidity higher than the alkalinity. Sometimes, the most challenging aspects of gardening are found working with hybrid hydrangeas [blue varieties] and rhododendrons. Several plants have these requirements – see below. Flowers Camellias – love acidic, well-drained soil; bloom in late winter or early spring. Azaleas – a type of rhododendron that flourishes in acidic soil. Rhododendrons – classic ericaceous shrubs with dramatic blooms. Hydrangeas (especially blue varieties) – their flower colour is pH-dependent; acidic soils produce blue blooms. Lupins – prefer slightly acidic to neutral soils. Primulas – especially Primula japonica. Calluna vulgaris (heather) – a true acid lover and great for low-maintenance beds. Shrubs Pieris japonica – attractive foliage and bell-shaped flowers. Skimmia japonica – suitable for shady, acidic borders. Leucothoe – evergreen with colourful foliage. Fothergilla – lovely autumn colour and bottlebrush flowers. Kalmia latifolia (mountain laurel) – prefers moist, acidic soil. Other Plants Ferns – many types, like the Athyrium or Dryopteris, enjoy acidic woodland conditions. Hostas – while adaptable, they prefer slightly acidic soil. Blueberries (Vaccinium spp.) require acidic soil (pH 4.5–5.5). Cranberries and Lingonberries – also Vaccinium family; acid lovers. Raspberries – prefer mildly acidic soils (pH 5.5–6.5). Strawberries – do well in mildly acidic soil (pH 5.5–6.5). |
| Blog Post: Keeping Blue Hydrangeas Blue – A Friendly Guide Blue hydrangeas are the jewel-toned stars of many gardens, admired for their stunning, almost surreal blooms. But you’re not alone if you’ve ever planted one expecting a brilliant blue and ended up with a patchy purple or blush pink. The secret to keeping them blue lies beneath the surface—in the Soil! Why Do Hydrangeas Turn Blue? Hydrangeas, particularly Hydrangea macrophylla and Hydrangea serrata, get their blue colouring from a perfect mix of acidic Soil and available aluminium. When the soil pH is low (below 6.0), and aluminium is present, the plant absorbs the aluminium, which reacts with pigments in the flowers to produce that brilliant blue hue. What Makes Them Lose Their Blue? If the Soil becomes more alkaline (pH above 6.5) or aluminium is lacking or locked up by phosphorus, the flowers will shift to pink or purple. This change can happen slowly, especially if you water with hard tap water or use the wrong fertiliser. How to Keep Your Hydrangeas Blue: Start With the Right Soil: Use ericaceous compost in pots or mixed into garden beds. This is designed for acid-loving plants. Water with Rainwater: Tap water in hard water areas like Kent can gradually raise soil pH. Rainwater is naturally softer and better for blue blooms. Add Aluminium: If your Soil is low in aluminium, you can use aluminium sulphate in spring and midsummer. Follow instructions carefully. Use the Right Fertiliser: Choose a low-phosphorus fertiliser (like 10-5-10). Avoid bone meal or lime-based products. Mulch Smartly: Pine needles, bark chips, and leaf mould help maintain acidity. Test Soil Regularly: Use a simple pH test kit to check if your Soil stays within the ideal range (pH 5.2–5.5). Do You Need to Replace the Soil? Not entirely. In pots, it’s best to top-dress each year (replace the top few inches of Soil) with fresh ericaceous compost. Every 2–3 years, report completely if the Soil is compacted or the plant outgrows the pot. In the ground, just amend the existing Soil with ericaceous compost or acidifiers as needed. Constantly digging up and replacing Soil can cause stress to the plant. Interesting Snippets: Did you know? White hydrangeas (Hydrangea arborescens and paniculata) can’t change colour based on Soil—they stay white regardless! Old wives’ tale: Burying rusty nails around your hydrangea won’t do much unless the Soil is already acidic enough to release the iron. Container tip: Blue hydrangeas are easier to control in pots—you can tailor the soil mix and avoid contamination from surrounding alkaline ground. Final Thoughts Keeping hydrangeas blue takes a little extra care, but it’s worth it for those bold blooms. Think of it as pampering your plant—they’ll reward you in colour. |
| Blue Hydrangea Fact Sheet Name: Hydrangea macrophylla / Hydrangea serrata Bloom Colour: Blue (in acidic Soil with aluminium), Pink (in alkaline Soil), Purple (in-between) Ideal Soil pH: 5.2–5.5 (acidic) Key Mineral: Aluminium (must be available and absorbable) To Keep Blue: Use ericaceous compost Add aluminium sulphate if needed. Water with rainwater Avoid lime and high-phosphorus fertilisers. Mulch with pine, bark, or leaf mould Maintenance Tips: Top-dress pots annually Repot every 2–3 years Test soil pH twice a year Feed with acid-loving plant fertiliser (low phosphorus) Fun Fact: You can shift the flower colour back to blue by adjusting pH and aluminium—but changes can take a full growing season to show. |
Earthly Comforts 
Thanks for this post on keeping Hydrangeas blue. Mine hasn’t bloomed yet, but when it does, I might be needing your instructions…
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