Best seed compost: Specialist blends for your seeds and seedlings

Everyone from green-fingered garden experts to newbie growers has experienced the disappointment of planting seeds only for nothing to grow. A common contributing factor to this failure to flourish is that ordinary garden topsoil, and even some quality general composts, can lack the necessary nutrient balance and specific physical structure that seeds need to grow and thrive. While there are no guarantees in the gardening game, starting your seeds, seedlings and cuttings off with the best seed compost you can find will ensure that they have the best possible start in life, and the best chance of reaching their maximum growth potential.

Once you’ve decided to invest in your garden’s future and load up on seed compost, there are still a few questions you’ll need to ask yourself. We’ll answer all the key questions in our buying guide, and below that you’ll find a list of our favourite seed composts, complete with mini-reviews breaking down their specific strengths and intended uses.

Best seed compost: At a glance

• Best value seed compost: Westland John Innes Seed Sowing Compost | £3.25
  
• Best peat-free seed compost: Melcourt Sylvamix Peat-Free Growing Medium | £24
  
• Best seed compost for versatility: Sinclair Modular Seed Compost | £24
• Best seed starter discs: Coco & Coir Seed Starter Pellets | £10

How to choose the best seed compost for you

Do I need a specific seed compost, or will regular compost do?

While it’s definitely possible to grow seeds with general or all-purpose compost, you may waste a lot of time, effort and seeds, and be disappointed by unpredictable germination and growth.

This is because seeds and seedlings require pretty specific amounts of nutrients, and the right soil texture, if they’re to germinate and grow to their full potential. Seed composts are specially blended to contain the correct balance of nutrients for early growth and healthy root formation. They help ensure that your seeds are neither under- or over-nourished – the latter of which might not even occur to rookie gardeners, but too many nutrients in the first stages of seed growth can be just as unhealthy as too few, and stall growth and germination similarly. Beyond nutrient composition, seed composts also tend to be much finer in texture when compared to general composts, whose coarse make-up can be unfriendly to fragile seedlings and delicate young roots.

READ NEXT: Our roundup of the best composts you can buy

What are some common compost ingredients and terms to be aware of?

While not exhaustive in terms of compost ingredients and soil amendments, this list below covers notable additions that appear in the products in our roundup.

• Peat: Similar to coal, oil and other fossil fuels, peat is formed from partially decayed, compacted animal and plant matter, and takes hundreds of years to accumulate in any notable quantity. Peat has long been favoured as a growing medium by gardeners due to its natural lack of pests and diseases, as well as its air, water and nutrient retention, which make it an excellent growing medium.
• Vermiculite: This light, flaky mineral comes pre-mixed into many composts. It helps with soil aeration and drainage by increasing permeability, while also improving water and nutrient retention due to its porous nature.
• Loam: While most composts don’t contain soil, certain types, such as John Innes composts, will contain loam. Loam soil is notable for its particular proportion of minerals: a roughly 40-40-20% blend by weight of sand, silt and clay, which give it excellent water retention and drainage, as well as making it easier to till. Loam soil is also generally rich in nutrients, and soil organic matter such as humus.
• Coir: Also known as coco coir, or coconut fibre, this material is made from the pith and/or outer husks of coconuts. It’s a long-lasting and effective growing medium, helping to keep soil aerated and moist, and is often used as a substitute for peat.
• Perlite: This light, granular material improves aeration and permeability, opening up the structure of soil or compost, with its low water retention helping with drainage. It’s also notably helpful for seed germination and growth.
• Barks: Mulches and barks are common compost amendments, and provide a range of benefits. They suppress the growth of weeds, regulate soil temperature and help retain moisture. Their natural decomposition also helps release nutrients, increasing soil fertility.
• Ericaceous: This is a term given to plants – such as rhododendrons, blueberries and azaleas – that are best suited to soil whose pH is between mildly acidic and neutral. For growing ericaceous plants, it’s important to make sure that your soil pH doesn’t register as alkaline, as ericaceous plants are unlikely to thrive in basic soil.

Should I be using a peat-based or a peat-free compost?

Peat compost is a garden staple, as it’s a highly effective medium for plant growth. However, using peat compost has been discouraged in recent years, with the UK government currently working toward phasing out its use in private gardens. This is due to environmental issues raised by peat extraction, as peat bogs are both large and effective carbon sinks, while also being home to several rare species of plants and animals, and so their rapid depletion raises serious biodiversity and sustainability concerns.

With this phasing out in mind, many growers and gardeners are opting for peat-free composts, due to them being not only more environmentally friendly, but increasingly available and easier to purchase as time goes on. While the above may sound like you’ll have to settle for a less effective product in order to protect the peatlands, this isn’t the case at all. Peat-free composts have different compositions to peat composts, but are generally just as effective at promoting growth and germination.

READ NEXT: Dig, shear and shape with our pick of the best garden tools

The best seed compost to buy in 2023

1. Melcourt Sylvamix Peat-Free Growing Medium: Best peat-free seed compost

Price when reviewed: £24 | Check price at Amazon

This growing medium contains coir, propagating bark, Melcourt’s proprietary Growbark, and enough nutrients and fertilisers to keep your seeds happy for around six weeks.

The pine-based Growbark employed by Melcourt is one of the key factors that makes this our favourite peat-free seed sowing compost, as it provides a lot of the advantages of peat in a much more eco-friendly package. The pine bark has a similar density to peat, is pest- and disease-free, stable in structure and non-degrading, and provides aeration and water balance. Beyond that, it’s low in nutrient content, making it ideal for controlling seed growth when used alongside other fertilisers and nutrients.

Recommended especially for vegetable seeds and cuttings, this compost’s solid but malleable structure makes it suitable for potting and modular containers as well as ground planting.

Key details – Size: 50L; Notable ingredients: Coir, propagating bark; Peat-free: Yes

2. Sinclair Modular Seed Compost: Best seed compost for versatility

Price when reviewed: £24 | Check price at Amazon

Sinclair’s compost is composed of peat, barks and perlite, with NPK (nitrogen, phosphates and potassium) nutrients added in low quantities to help germination. This product is designed for filling cells in modular seed trays, which it is well suited to, but also works a treat in other areas of the garden.

Suitable for growing both ericaceous and non-ericaceous plants, this versatile growing medium retains moisture well and has a free-flowing texture, making it suitable for everything from sowing seeds and helping cuttings establish roots to growing transplants.

Customer feedback for this growing medium is notably positive, with many users stating that it helped their seeds thrive where other products had failed completely.

Key details – Size: 75L; Notable ingredients: Peat, barks, perlite; Peat-free: No

3. Westland John Innes Seed Sowing Compost: Best value seed compost

Price when reviewed: £3.25 | Check price at Homebase

Westland’s John Innes Seed Sowing Compost is composed of loam, peat, horticultural grit and vermiculite as well as additional nutrients. This compost contains sufficient nutrients to sustain your seeds for the first three weeks after planting, with the combination of soils, grits and additives providing an optimal texture and structure for germination and root formation. The addition of vermiculite helps with moisture retention, while also insulating your seeds, protecting them from changes in temperature and humidity. This seed sowing compost is also recommended for establishing cuttings.

Once your seeds have germinated, Westland also has a range of follow-on composts designed for every stage of growth. The next one you’ll need is the Young Plant compost, which is geared towards plant establishment.

Key details – Size: 10L; Notable ingredients: Peat, loam, vermiculite; Peat-free: No

4. Coco & Coir Seed Starter Pellets: The best seed starter discs

Price when reviewed: £10 | Check price at Coco & CoirFor smaller gardens, more compact setups or just avoiding hassle, seed starter discs are a very handy option that are simpler to use and more easily stored than large bags of compost.

These seed pellets from Coco & Coir are peat-free and biodegradable, with the substrate itself made from coco coir and nutrients. To start growing, you simply need to add a small amount of water to the pellets, placing your seed or cutting into a small notch on top.

Because of their size and structure, the pellets are also easy to plant out into larger pots or patches of garden once they’ve begun to establish themselves.

Key details – Size: 50pc; Notable ingredients: Coco-coir; Peat-free: Yes

Check price at Coco & Coir