The Problem With Peat-Free Compost (And How to Fix It)

Just last week, I reported on research from Stockbridge Technology Centre (STC) that found many peat-free composts delivered unacceptable results.

Of course, I wanted to know more – and get tips for gardeners on how to select and get the most out of these composts. 

Julian Davies, Agronomist and Commercial Director of STC kindly agreed to answer my questions, offering invaluable insights and practical advice.

Julian’s research revealed that almost 20% of peat-free composts produced unacceptable results four weeks after sowing.

In a follow-up project running from 2024 to 2025, the failure rate increased to 40% four weeks after planting. 

However, with a few strategic improvements, the team were able to dramatically improve results. 

The real reason for poor performance 

Despite the harm that peat-harvesting causes to the environment, peat is prized for its structure and its ability to retain water. 

So you’d expect that peat-free compost would lack these two benefits. 

Surprisingly, that wasn’t the case: 

“Interestingly the structure and water holding ability are not the reason for poor performance.

“In my view it is the nutrient content and possibly the shelf life of the products.”

Julian’s team found two simple ways to improve the performance of peat-free compost. 

1. Adding NPK fertilizer before planting. This reduced unacceptable results from 40% to just 6%. 
2. Using liquid feed. Julian noted that:

“…if we had started earlier [with liquid feed] then we would probably have got better growth but that would have meant starting immediately post planting – which I think  should not be expected.”

“There should be sufficient nutrients in the compost for the first 4 weeks.”

It’s important to note that not all the peat-free composts performed badly – some were rated as good or excellent. 

What makes the difference? 

“The inclusion of CRF (controlled-release fertilizer) appears to be one reason why the peat-free composts are performing well.”

Peat-free composts have been around for decades. I remember my father complaining about them when I was a child (don’t ask how long ago!)

So with years of research and growing demand for alternatives, are they finally getting better?

Julian’s response was mixed:

“Compared to 20 years ago, they are better overall, but there is still a lot of variability in performance between products—and even between batches.”

Once again, the problem isn’t structure or water retention. The real issue remains nutrient content and compost shelf life.

While many plants adapt well, some are more sensitive to peat-free compost, especially when made from green waste.

Two notable problem plants are:

• Tomatoes
• Dahlias

Both are highly sensitive to low levels of herbicides, which can sometimes be present in green compost.

Can gardeners make quality peat-free compost at home? 

Julian noted that he didn’t have much experience with home made compost beyond mulching, but was optimistic that high-quality potting compost could be made at home. 

His key tips were: 

• Allow compost to mature for longer
• Sieve compost before use
• Be prepared to either add fertiliser or liquid feed from planting or seed emergence

Takeaways for gardeners

Sometimes when we get poor results from a sowing, it’s tempting to think that we are at fault.

However, research from STC, which chimes with experiments by other gardeners, shows that’s not always the case.

Compost quality can vary between brands, and other (less formal) experiments have found that they can vary between different batches from the same brand.

Fortunately, STC research gives us some tips:

1. Look for peat-free compost with added slow-release fertiliser.
2. If you have a batch of compost which is performing slowly, add slow-release fertiliser before planting another batch.
3. To rescue plants which are growing poorly, add liquid fertiliser.
4. Don’t be scared to make your own potting compost – after all, it can often be better than store-bought compost.

I’ve also previously managed to rescue a friend’s chilli plants, which were struggling in seed compost, by mixing worm castings with regular compost and repotting the plants.

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