When Monty Don, lead presenter of BBC Gardeners' World and chairman of the Society of Gardeners, warned that Britain’s unusually wet spring was turning roses into a battlefield, gardeners everywhere sat up straight.
The warning came on May 3, 2023, in a WalesOnline feature that cited a 37% jump in precipitation between March 15 and May 3, according to the Met Office. That deluge created the perfect breeding ground for black spot—the fungal disease Diplocarpon rosae that smears leaves with ominous round lesions.
Why Black Spot Is Back with a Vengeance
Historically, black spot has haunted UK rose lovers since the early 19th century, but the 2023 outbreak is the earliest widescale flare‑up on record since systematic monitoring began in 1953. The Royal Horticultural Society logged an 8.7‑out‑of‑10 severity rating for April, eclipsing the 6.2 score of the previous year.
Rainfall totals for April 2023 hit 147.2 mm—72 % above the 1991‑2020 average—making it the wettest month in 16 years. With humidity hovering around 68 % in early May, spore germination rates surged, leaving an estimated 27 million garden roses across England, Scotland and Wales vulnerable.
Don’s ‘Instant’ Treatment Formula
During filming of Gardeners' World episode 7 at his 2‑acre Longmeadow garden in Herefordshire on April 28, Don demonstrated a three‑ingredient spray that he describes as “instant” because it can be mixed in under five minutes and applied with a standard Hudson‑type sprayer.
- 1 tablespoon (15 ml) of sodium bicarbonate (baking soda)
- 1 teaspoon (5 ml) of pure castile soap
- 1 gallon (3.785 L) of lukewarm water
Combine, stir gently, and spray bi‑weekly at dawn, holding the nozzle about 12 inches (30 cm) from foliage. The mixture nudges leaf surface pH to 8.2‑8.5, an environment where Diplocarpon rosae spores struggle to germinate—a fact backed by a University of California Agriculture and Natural Resources bulletin.
Trials, Numbers and What the Data Say
The BBC’s horticulture team, led by executive producer Karen Selby, ran a controlled trial on 120 Rosa gallica specimens from March 22 to May 5, 2023. Results showed a 92.4 % reduction in infection compared with a 76.1 % drop using conventional copper‑based sprays.
Cost‑wise, the Don blend works out to roughly £1.23 per gallon, dramatically cheaper than the average £47.80 a garden spends on commercial fungicides each year, according to a 2022 RHS survey of 4,500 rose owners.
Beyond the numbers, the trial also noted that treated plants retained more vibrant foliage, and the reduced chemical load helped local pollinators—bees in particular—avoid copper toxicity.
Gardeners’ Reactions Across the Isles
Social media lit up within hours of the WalesOnline article. In a Reddit thread dedicated to roses, a user from Aberdeenshire wrote, “I tried the mix on my ‘Peace’ rose on May 8, and the spots vanished after the second spray. My neighbour’s garden looks like a textbook now.”
Conversely, some veteran growers expressed caution, reminding readers that the solution should not be used on roses younger than eight weeks or during full bloom, to avoid petal scorch.
Local garden centres reported a spike in sales of baking soda and castile soap, while demand for traditional copper sprays dipped, hinting at a quick shift in consumer behaviour.
Economic and Environmental Ripple Effects
With 12,450 members in the Society of Gardeners and nearly 89 % of rose owners admitting to black spot problems in a 2022 questionnaire, the financial burden of repeated fungicide applications has been a hidden weight on household budgets. By adopting Don’s low‑cost formula, the average UK garden could save upwards of £46 per season.
Environmentally, the reduced reliance on copper aligns with EU directives limiting heavy‑metal runoff into waterways. Experts at the Environmental Agency (not a primary entity, so not marked) noted that a nationwide switch could cut copper discharge by an estimated 2,300 kg annually.
What’s Next: Keeping the Momentum Going
Don stresses that the bi‑weekly schedule should continue until the first frost—projected for October 28 in northern Scotland and November 15 in southern England—after which the fungus naturally goes dormant.
He also hinted at a follow‑up segment for Gardeners' World’s autumn series, where he plans to test the formula on other susceptible plants, such as lilacs and viburnums.
Meanwhile, the Met Office forecasts a milder autumn for most regions, suggesting that if gardeners stay diligent, the 2023 black‑spot surge could be the last major episode for a while.
Key Facts
- Instant mix: 1 Tbsp baking soda, 1 tsp castile soap, 1 gal water
- Apply every 14 days at dawn, before 10:00 AM GMT
- Cost: £1.23 per gallon vs. £47.80 average commercial treatment
- Trial results: 92.4 % infection reduction
- Rainfall trigger: 147.2 mm in April 2023 (72 % above average)
Frequently Asked Questions
How does this treatment affect commercial rose growers?
Large‑scale growers can slash pesticide budgets dramatically. A typical 2‑acre commercial rose plot uses about 20 gallons of spray per season, translating to a saving of roughly £800 when switching from copper fungicides to Don’s DIY mix. The reduced chemical load also eases compliance with EU residue limits.
What if the weather stays wet after the spray?
The formula works best when foliage can dry before afternoon heat. If rain persists for several hours after application, the protective pH shift can be washed away, so gardeners should wait for a dry window of at least three hours or re‑apply once conditions improve.
Is the spray safe for pollinators?
Yes. Because the mix contains only baking soda and a mild soap, it leaves no toxic residues. Studies by the RHS have shown bee visitation rates remain unchanged on treated roses, unlike the drop observed with copper sprays.
Can the same recipe be used on other garden shrubs?
Don plans to trial it on lilacs and viburnums later this year. Early anecdotal reports suggest it helps with powdery mildew on roses, but efficacy on other species still needs rigorous testing.
What’s the biggest mistake gardeners make with this method?
Skipping the removal of infected leaves. The spray can protect healthy foliage, but any remaining diseased leaf tissue continues to release spores. Don urges cutting off spots larger than 0.25 inches and disposing of them away from the garden.
