Peat-free Compost Trial with Tom Brown

Posted on 14th April 2025

We regularly get asked which peat-free compost to use, as visitors desperately grapple to find a product to overcome their disappointment from composts previously used. We’re looking for a peat-free compost that not only will encourage good germination but will also deliver reasonable plant growth for larger plants such as bedding. It's well documented that we need to feed peat-free composts to get the best results, but seedlings require very little fertiliser until they become more established, so this trial of compost relied purely on water with no supplementary feed. As the seedlings and plants develop, they will indeed need feeding, but the purpose of this trial was to see initial germination and initial growth within the first six weeks of being potted, based purely on how the plants reacted to the compost.

Process

There were three parts to this trial, for each compost we took a retail packet of Cosmos and tomato seed and sowed them into a half tray of the compost, resulting in two half trays of seedlings (all being well). Cosmos seed is a larger seed and arguably less sensitive to fluctuations within the compost, but tomato seeds require more warmth and as a smaller seed should give us more variable results, and I thought would be more of an indicator. As well as germinating these two garden stalwarts, a spider plant, a calibrachoa (a type of petunia) and a coleus were potted into the compost. The objective was to find a compost that would not only give good results through germinated seedlings but also provide enough substance to encourage strong plant growth within the first six weeks of potting a young plant.

It is worth noting that although we tried to replicate the same conditions that you’d experience at home, this trial was specific to our conditions, at a certain time of year, with specific plants. Many composts that we have not recommended may work perfectly well in different conditions, or you may have experienced favourable results at home. Our findings are based on our experience with conditions that were standardised as much as practically possible.

What we discovered

Plants and seeds were given a consistent environment, as we tried to create uniform conditions to give us the most accurate results. This was by no means a scientific trial, but we did try to replicate the conditions that most gardeners would grow their seeds and plants.

As you can see from the results table there were a few composts that were borderline in their performance, but I have to say, the difference was quite stark in most cases; either those composts being a group that performed well, or those that were thoroughly disappointing. It's worth saying that some of the composts might be perfectly suitable for larger plants, for example if you're growing in hanging baskets or large containers during the summer with a weekly supplementary feed.

The Results table

Product	No. of Cosmos seedlings germinated from retail packet	No. of tomato seedlings germinated from retail packet	Plant growth
Levington Multipurpose	18	17	Good
Verve Multipurpose	9	1	Poor
Miracle Grow All Purpose	12	0	Fair
Westland Multipurpose	15	0	Poor
Bathgate Multipurpose	16	29	Good
Carbon Gold Organic Biochar Seed Compost	17	21	Fair
Westland Seed Compost	13	27	Good
Westland Organic All Plant Compost	23	16	Fair
Rocket Gro Multipurpose	6	0	Poor
Dalefoot Wool Compost for Seeds	14	4	Fair
Natural Grower Organic Compost	13	0	Poor
Fertile Fibre Organic Seed Compost	18	3	Good
Growmoor Multipurpose	14	27	Fair
Greenbrokers Potting Compost	21	14	Good
Mr Fothergill’s Seed Compost	16	18	Good
Melcourt Sylvagrow Multipurpose (Used at West Dean for potting on)	16	2	Good
Petersfield Supreme Multipurpose (Used at West Dean for sowing seed)	19	23	Good
Coco Coir	14	9	Fair
Sarah Raven Compost	19	14	Good

Watering

With all the different seed trays and plants that were grown in this trial, we notice that each compost required a reasonably individual watering technique to get the best results. Some compost dried out very quickly, others held on to moisture and the only way to determine whether the trays or plants needed watering was to lift the tray and assess the weight. The saturated composts were heavier and when those composts became lighter in weight, needed hydrating from below. Like most seedlings it's best not to water from above as this causes disease and rot, with all the seedlings being watered in a tray of water, meant that they could absorb the moisture from underneath, keeping the seedlings dry at all times.

Different compost for different jobs - fine vs coarse

The majority of compost that gave good results in seedling germination, were much finer in their texture. The very coarse compost that had a high wood content proved to be less suitable for seed sowing. With peat-based composts we became used to a particular product being suitable for sowing seeds and then potting on young plants. In our experience, peat free composts are suited to certain plant growth stages, different composts for different jobs. Seeds are expensive to buy and therefore I think it's justifiable to spend a little more on a quality seed compost that will give you good germination. It's a false economy for your pocket and for your time to cut corners and buy cheap compost that will then result in poor germination.

Feeding

None of the seedlings or plants in our experiment were fed in any way and relied purely on water or the fertiliser that already existed within the bags of compost. This gave us a clear indication of how the compost effected the plants. It is worth noting that you want to avoid buying peat free compost that has been sat around in the garden centre for a long period of time and become saturated, as fresher compost gives a better performance. Once plants and seedlings have been grown in compost for a period of four to six weeks, then start feeding your seedlings with a seaweed-based fertiliser at 10% of the recommended dilution rate. Larger plants can be fed with a seaweed-based fertiliser once a week, switching to a potash-based fertiliser such as tomato feed later in the season when those plants develop flowers.

Compressed composts

An area of this trial that we were incredibly intrigued by was the use of compressed composts. These composts had been dried out and vacuum packed for us to hydrate upon receipt of the compost. This concept seemed quite alien, and to be honest we were quite sceptical, but the practicalities of these composts have great merit. All the compost was purchased online to be delivered, but for those who struggle to carry a heavy bag of compost around, or upstairs, then I'd recommend that you look into these compressed composts, as they are lightweight until you get them to their destination, where you can duly hydrate them to grow your seedlings. As it turned out, not only were they easy to use and practical, but they also gave good results when it came to germinating seeds and growing young plants.

Prick seedlings out quickly to avoid stalling

Our tip would be that some of the peat free composts did encourage a good rate of germination but once seedlings had germinated, they stalled and stopped growing. As a rule of thumb, with your peat free composts, it is worth pricking out and potting on your seedlings when they are large enough to handle. If you find that you achieve good germination but then the plant seemed to stop growing, move them on into fresh compost and start a feeding regime to keep them actively growing.