Protecting crops without chemicals

From baking powder to seaweed, organic growers have a treasure chest of unlikely techniques to protect crops from pests and disease without using artificial chemicals.

Spring is a time of year when the weather can be more unpredictable than usual, but organic farmers don’t have the luxury of spraying artificial chemicals to control pests and disease. Instead, they use an eclectic armoury of tools ranging from baking ingredients to seaweed to boost the plant’s natural resilience.

A little-known fact is that seeds naturally harbour plant disease pathogens on their surface. It depends on the weather and soil conditions as to whether these pathogens are activated and the plant becomes infected, but by then the plant is already growing and it can be too late.

For non-organic growers, (known as conventional), seeds are typically treated or coated with a synthetic chemical to get rid of the pathogens and remove the risk of disease. Organic growers don’t have the same option, but a new trial hosted on organic veg box company Riverford’s farm in Devon has tested a new technique with the same effect.

A special crop of Swiss chard and spinach seeds that had been doused in hot water were tested against a crop without the hot water treatment, to see if it reduced the amount of leaf spot. Leaf spot is a fungal disease leading to black marks on leafy veg, meaning that means pickers have to spend longer selecting leaves. Labour is already one of the highest costs for any organic farmer, who must weed by hand without the help of powerful synthetic herbicides, so any additional costs are a significant burden.

Chard leaf spot
Leaf spot is a fungal disease that causes black marks on leaves

“You don’t want the crop covered in leaf spot, there is a certain tolerance but once it goes above that guys have to start picking them out,” explains Riverford farm manager, John Richards. “Our pickers can harvest around 15-20 kg of spinach an hour, but if that goes down to 10 there is a real financial impact.”

Overall, the hot water treated seeds did lead to less leaf spot in the crop, although this may have been helped by last summer’s extreme heat and low disease pressure.

Riverford’s polytunnel manager, Ed Scott, explains: “The field results from last year were promising and found some benefits – the hot water treatment looks to be a useful tool in helping control leaf spot and improving crop yield. This also helps with labour savings as it is quicker to harvest the crops if there is less disease.

“However, 2018 was a particularly dry year with low levels of disease in general, so we are looking forward to comparing results to further trials this year.”

More testing is also required to make sure no beneficial bacteria is killed off the seed’s surface at the same time as the leaf spot fungus.

Chard field
Chard seeds were treated with hot water in a trial

It’s not just organic farmers who could benefit from hot water seed treatment if it is proved effective. The number of agri-chemicals that are available to all growers is reducing as concern about their safety and environmental impact heightens, hence the renewed interest in alternative crop protection. Preventing disease before germination without the use of harmful chemicals could well be a win-win for all.

Four more helping hands from nature…

Taking a tea break

Comfrey is said to work as a natural fertiliser. Image credit Dean Morley. 

Comfrey tea has an anecdotal reputation for being a good fertiliser for fruiting or flowering crops, such as cherry tomatoes and courgettes, but so far there has been little scientific evidence to back this up. A new trial hosted by Riverford and other growers in the south west tested the theory by applying comfrey tea to trial crops during the 2018 season, to monitor whether additional nutrients lead to tastier fruit, and a deeper colour. Both the hot water and the comfrey tea trials at Riverford were supported by the non-profit Innovative Farmers network, which enables farmer-led research into new techniques.

A baker’s best tip

After almost 20 years’ growing organic apples, Paul Ward has a few tricks up his sleeve. And if one of them sounds similar to a store cupboard staple, that’s because it is. Food-grade bicarbonate of soda, or baking powder, as it’s commonly known, is sprayed onto apple tree leaves if there is a risk of disease spores spreading. “One of the problems we face as organic growers are fungal pathogens, which live naturally on leaves and can cause dark marks on the skin of apples,” explains Ward. “If there is a high risk of spores, that’s when we might spray the leaves with a benign biological control, like bicarbonate of soda, also known as baking powder! It’s actually a big part of our armoury.”

Solving it with seaweed

Seaweed is emerging as one of those magic materials that seems to be useful for everything. It can be eaten as a nutritious protein, used as a plastic alternative in packaging, and even as a natural fertiliser for fruit trees. Organic apple grower Paul Ward uses a seaweed spray, mixed in tanks on his farm, on the leaves of his apple trees to improve their natural resilience. “It’s a great biostimulant and makes the plant stronger and with better quality leaves,” he says.

Good old green manure

A healthy plant is far more likely to resist disease, and making sure the soil is as rich and fertile as possible is the best way to improve its inherent resilience. Adding green manure (made from composted garden trimmings), as well as organic chicken pellets and compost made from leftover food scraps, adds vital nutrients and minerals into the soil, giving the plants the best innate strength possible.


Leave a Reply

  1. Whilst I don’t advocate the use of chemics, the HWT seeds in chard is not new. In fact it’s standard for nearly all chard seed companies. Yes it has benefits for sure; the best way to have a clean crop is to start clean of course. The dry year last year for certain was a leading factor to reducing pressure and symptoms (as stated).
    There are no organic treatments as additions to the HWTs which I’m sure you will be able to try out in the 2019 season.

    1. I see a mistake in my message, **There ARE organic treatments available as additions which you are aware of.


In case you missed it

Receive the Digital Digest

Food, Farming, Fairness, every Friday.

Learn more

About us

Find out more about Wicked Leeks and our publisher, organic veg box company Riverford.

Learn more