Let’s cut to the chase: if you grow in pots and containers in the UK, the compost you pick matters more than you think. You’re juggling watering, root space, occasional neglect, and weather swings — and the wrong medium will punish you. Here’s a pragmatic, slightly grumpy but trustworthy guide that compares the real contenders: peat-free commercial mixes, John Innes No.3, and homemade compost. I’ll give you expert-level takeaways, a decision matrix, and a short interactive quiz so you can decide without faffing about.
1. Establishing the Comparison Criteria
Before picking a winner, we need a clear set of criteria that actually matter for container growing. From your perspective — limited root volume, need for regular watering, and the desire for healthy crops/flowers — judge options by:
- Water retention (how well the mix holds moisture between waterings)
- Drainage and aeration (prevents root rot, supports root growth)
- Nutrient availability and longevity (initial fertility and whether you must feed)
- Structure and compaction over time (does it slump or stay friable?)
- Salt and contaminant risk (particularly with green-waste or coir-based mixes)
- pH suitability for typical pot plants (vegetables, ornamentals, acid lovers)
- Weight (important for hanging baskets or roof terrace pots)
- Sustainability and peat content (peat is being phased out; do you care?)
- Cost and convenience
We’ll apply these consistently so you can compare apples to apples — or at least tomatoes to tomatoes.
2. Option A — Peat-Free Compost
What it is
Peat-free composts replace peat with materials such as coir (coconut fiber), green compost (recycled garden/greens), wood fiber, bark, or composted bark. Brands in the UK have improved a lot in the last decade — but not all mixes are equal.
Pros
- Environmentally preferable: reduced peat extraction, better for bog habitats
- Good water retention when coir or wood fiber is used — useful in hanging baskets
- Some mixes are lightweight — less strain on balconies and elevated planters
- Often comes ready to use for container plants (no extra loam needed)
Cons
- Variable quality and nutrient levels — many peat-free mixes are low in NPK and need feeding
- Coir can be saline or potassium-heavy unless buffered — risk to seedlings and sensitive plants
- Green-waste compost can be inconsistent, may contain weed seeds, pathogens, or high soluble salts
- Some mixes break down faster and compact in containers unless amended with grit/perlite
In contrast to John Innes, peat-free mixes tend to be lighter and more focused on water retention rather than mineral content. Similarly, they can be kinder to the planet — but only if you choose a reputable, well-processed product.
3. Option B — John Innes No.3
What it is
John Innes No.3 is a classic loam-based compost formulated in the mid-20th century for pot-grown fruit and vegetables. It uses sifted loam, sharp sand, and peat: the No.3 is the richest in the John Innes range (No.1, No.2, No.3).
Pros
- Consistent structure and nutrient content — contains loam so nutrients are available without immediate feed
- Good drainage with an open tilth due to the sand component; roots like it
- Reliable pH levels and low salt — predictable performance for vegetable/container cropping
- Holds together well in larger pots, less prone to washing out
Cons
- Contains peat (unless you find a peat-free John Innes analog) — environmental downside
- Heavier than many peat-free mixes, so not ideal for hanging baskets or very large, temporary planters
- Can crumble and dry out quicker in small pots unless you add water-retentive amendments
- May need extra organic feed for long-season crops
On the other hand, John Innes No.3’s consistency and mineral content make it a predictable choice for tomatoes, peppers, and container vegetables where you want a steady base fertility and good drainage. It’s the purist gardener’s go-to if peat concerns aren’t your priority.
4. Option C — Homemade Compost
What it is
Your own compost made from kitchen scraps, garden cuttings, cardboard, brown material, and manure (if available). If you’re doing it right, it’s a living, nutrient-packed material — but that phrase “doing it right” is the catch.
Pros
- Extremely sustainable and cheap; reduces your household waste
- If well-matured, it’s biological, nutrient-rich, and feeds the soil life — great for pot plants when mixed correctly
- You control inputs so you can avoid contaminants and build quality over time
- Mixes well with other substrates to add structure, nutrients and biological activity
Cons
- Highly variable: immature compost can be phytotoxic, steal nitrogen, or harbor pathogens and weed seeds
- Often too fine and compacting for containers unless combined with grit/perlite/loam
- Time and space required; not instant gratification
- Risk of slugs, vine weevil, or other pests if not properly processed
Similarly to peat-free mixes, homemade compost’s usefulness depends on processing. If your compost is three-month hot composted and sieved, it’s a great amendment. If it’s semi-rotted and full of twigs and vegetable peel, expect surprises.
5. Decision Matrix
6. Clear Recommendations
Right — if you want a straight answer without nuance (because sometimes you do):
- If you prioritize sustainability and you’re willing to choose carefully: go peat-free but pick a proven brand, and always mix in grit or perlite for aeration. Add slow-release feed or regular liquid feeding.
- If you want predictability and you’re growing vegetables or long-season crops: use John Innes No.3, or a similar loam-based mix. In contrast to many peat-free mixes, it gives reliable structure and mineral content, but it’s heavier and peat-based.
- If you compost well at home: use your compost as an amendment, not the sole medium. On the other hand, if your homemade compost is immature, don’t use it alone — it will punish you.
Practical mixes I use and recommend
From your point of view, these three mixes cover most needs:
- Peat-free shrub/plant mix + 10-20% grit/perlite + slow release fertilizer (for mixed patio pots)
- John Innes No.3 + 10% composted bark or coir + slow release feed (for tomatoes and heavy feeders)
- 50% mature homemade compost + 50% sharp sand/loam/grit (for large planters and veg beds in containers)
Expert Insights (yes, the nitty-gritty)
• pH matters: Most peat-free mixes tend to be near neutral; John Innes is slightly alkaline. If you’re growing ericaceous (azaleas, blueberries), buy an ericaceous peat-free mix or acidify with ericaceous fertilizer. Do not assume “peat-free” = “suitable for acid lovers.”
• Coir buffering: Coir often arrives high in sodium and potassium. Good suppliers buffer coir with calcium; cheap bags may not. If seedlings show burning or stunted growth, leach the substrate thoroughly before potting.
• Salt soluble nutrients: Green-waste compost may contain high soluble salts, which harm seedlings. Use green-waste as an amendment, not as the sole medium. In contrast, John Innes has low soluble salts and is safe for sensitive transplants.
• Structure management: Containers lose structure faster than garden beds. Top up annually with fresh compost or replace part of the medium. Similarly, don’t be stingy with grit or chunky bark in mixes for large containers to maintain https://gardenadvice.co.uk/newsandarticles/transforming-outdoor-spaces/ aeration.
• Fertilizer routine: Whatever base you choose, containers need feeding. Use a good liquid feed fortnightly in growing season, and/or add a slow-release granule at potting. John Innes reduces the need for immediate feeding; peat-free usually requires more frequent feeding.
Interactive Self-Assessment: Which Compost Suits You?
Answer these four quick questions — keep score and match to the result below.
- A) Yes — strongly (3 points)
- B) Somewhat (1 point)
- C) Not really (0 points)
- A) Yes — tomatoes, peppers (0 points)
- B) Mostly ornamentals (2 points)
- C) Mixed (1 point)
- A) Yes — mature, hot-composted (3 points)
- B) I try, but it’s hit-and-miss (1 point)
- C) No (0 points)
- A) Yes (0 points)
- B) I don’t mind feeding (2 points)
- C) I actively like feeding and fiddling (1 point)
Scoring:
- 9–12 points: Peat-free + homemade amendments. You’re eco-minded and confident — pick a high-quality peat-free mix, buffer coir if needed, and use your mature compost as part of the blend.
- 5–8 points: Consider a hybrid approach. Use John Innes No.3 for heavy feeders and your peat-free mix for hanging baskets and ornamentals. Mix in your well-matured compost where appropriate.
- 0–4 points: John Innes No.3 or a similar loam-based mix is likely your safest bet. It’s predictable and reduces the risk of seedling shock or nutrient volatility. Add feed as necessary.
How to Make Homemade Compost Work for Pots (Practical Steps)
- Only use fully cured compost: at least 6 months, preferably a year, and pass it through a 10–15 mm sieve for containers.
- Mix ratios: Don’t use more than 30–50% home compost in a potting mix. Better formula: 50% mature compost + 30% sharp sand/loam/grit + 20% coir or bark for structure.
- Hot compost if possible: 60–65°C for a few days to kill pathogens and seeds. If you can’t, mature cold compost longer and screen well.
- Test for salts: if plants show leaf-tip burn, flush the mix with water and reconsider the compost source.
- Add slow-release fertilizer if the compost’s nutrient profile is unknown.
Final Words — The Short List
From your perspective, you want something that works reliably, is relatively sustainable, and doesn’t require constant panic-watering. Here’s the short list you can act on tomorrow:
- For reliable veg and long-season containers: John Innes No.3 + feed. It’s predictable and low-risk.
- For eco-conscious gardeners and hanging baskets: high-quality peat-free mix + grit/perlite + feed. Read the label: look for buffered coir and low-salt green compost.
- If you compost: use your compost as an amendment, not a full potting medium, unless it’s exceptional quality and mature.
There. No fluff. You’re better off now than you were two minutes ago. If you want, tell me what you grow (balcony tomatoes? patio herbs? shrubs in large planters?) and I’ll give you a precise potting recipe and feeding schedule for that exact setup.
