Comfrey versus nettle: which makes the best plant feed?

Since the pioneering work of our very own Lawrence Hills, comfrey has earned a reputation as the ideal feed for fruiting crops such as tomatoes due to its high potash content. Nettle feeds, meanwhile, are often recommended for leafy crops due to their nitrogen levels.

To test whether this advice still rings true, we conducted a citizen science experiment comparing homemade comfrey and nettle feeds grown side-by-side under similar conditions.

Putting the feeds to the test

The experiment compared 19 paired samples of comfrey and nettle liquid feeds from a range of locations. At each site, both plants were grown in similar conditions, allowing a direct comparison of their nutrient content.

The results revealed some differences between the two feeds, but perhaps not the ones many gardeners would expect.
Nettle feeds contained significantly more nitrogen than comfrey feeds. Nitrogen supports leafy growth, making it valuable for crops such as lettuce, spinach and brassicas.

Comfrey feeds contained slightly more potassium (potash), which is associated with flowering, fruit development and flavour. However, the difference between the two feeds was small and not statistically significant.

Perhaps most surprisingly, nettle feeds also contained more phosphorus than comfrey feeds. Phosphorus plays an important role in root development and overall plant health.

Good news for tomato growers

The most important finding from the study was that both comfrey and nettle feeds contained nutrient levels that make them suitable as general-purpose plant feeds, including for tomatoes.

This challenges the common belief that nettle feeds are unsuitable for fruiting crops. In fact, the nettle samples analysed contained high levels of potassium and compared favourably with both comfrey feeds and commercial tomato fertilisers.

The findings also supported earlier work carried out by Lawrence Hills, whose analyses showed that comfrey feed had nutrient levels remarkably similar to modern tomato feeds.

Making your own liquid feed

The study focused on feeds made by steeping leaves in water to create a ready-to-use liquid fertiliser. One practical outcome of the research is that these homemade feeds generally do not require further dilution before use, as their nutrient concentrations are comparable to commercial feeds when applied to plants.

For the richest nutrient content, younger leaves are generally the best choice. If you regularly harvest comfrey, it is also worth feeding the plants occasionally, as repeated cutting removes nutrients from the growing area.

So, which is best?

Comfrey remains an excellent choice, particularly as it also provides valuable pollen and nectar for bees. But if nettles are more readily available in your garden, there is no reason not to use them.

Whether you're growing tomatoes, courgettes or leafy greens, both plants can help create a low-cost, environmentally friendly liquid feed.