Bicarbonate of soda in winter: remove moss and weeds from paving joints

Plenty of homeowners put up with green-stained joints between block paving, slabs and patio stones for months. Then spring arrives and you spend hours on your knees scraping out moss, blasting the area with a pressure washer - only for it to look much the same again a few weeks later. Professionals take a different approach: they use the quiet winter period and rely on an everyday ingredient found in most kitchens.

Why moss and weeds are vulnerable in winter

In December the garden can look as if it has shut down: beds are bare and shrubs carry few leaves. Between paving slabs, though, it’s a different story. In those narrow joints, roots, spores and seeds are surprisingly well protected from cold snaps and frost. As soon as the first milder days appear, they kick back into gear and turn joints into slippery little strips of growth.

That’s exactly why acting in mid-winter can pay off. In January or February, plants are under stress and their metabolism slows. Disturbances hit far harder than they do during peak growth. If you intervene at this point, you break the cycle before moss and weeds can properly get going.

Many people reach for boiling water or vinegar. Both come with drawbacks: near-freezing or waterlogged slabs can be more prone to micro-cracking if you pour on very hot water, and vinegar often leaves roots and seeds alive. A simple mineral salt from the cupboard tends to work far better.

The overlooked kitchen powder for moss

The ingredient is plain bicarbonate of soda, commonly sold as “baking soda”. It’s a mineral salt that doesn’t poison plants in the way typical chemical weedkillers do - instead, it dries them out.

Bicarbonate of soda draws water out of the cells of moss and wild weeds, weakens the roots and, at the same time, disrupts germination conditions in the joints.

What happens is an imbalance at the plant surface and around the roots: water moves out of the cells, and the plant effectively desiccates from within. At the same time, the pH of the jointing material rises slightly. Many mosses and common joint weeds don’t tolerate that well, so they germinate more poorly or fail to return.

Winter adds another advantage: there’s usually plenty of moisture around, whether from dew, mist or fine drizzle. The mineral dissolves gradually and can seep deep into the cracks - without any exhausting scrubbing.

How the dry winter treatment works

Professionals often start with a straightforward dry application. It’s particularly suitable if the patio is only lightly greened over, or if you want to prevent bigger problems later.

Step-by-step guide to drying out moss and weeds

• Clear loose debris first: sweep away leaves and light dirt with a broom; the joints don’t need to be spotless.
• Measure the bicarbonate of soda: allow about a small handful per linear metre of joint - roughly 20 grams.
• Scatter the powder: sprinkle it lightly over the joints, taking care not to drop it onto beds or the lawn.
• Brush it into place: use a scrubbing brush or stiff broom to work the powder down into the joints so it contacts soil and roots.
• Don’t rinse afterwards: don’t pour water on. Natural winter damp will dissolve it and carry it deeper.

Over the following days, the salt dissolves slowly. It trickles down, reaches leftover root material and knocks it back while it’s already weakened. Ideally, by spring the joints are close to bare and you only need to sweep out any loose residue.

Liquid treatment for heavily greened areas

If the patio is already thick with moss, dry scattering often isn’t enough on its own. In that case, a water-based solution adds extra impact - especially where you need to shift visible cushions of moss and dense tufts of grass in the joints.

Mix for a more natural “liquid treatment”

For a bucket or larger watering can, the following blend works well:

• approx. 950 millilitres of water (warm, but not boiling)
• 2 to 3 tablespoons of bicarbonate of soda, stirred thoroughly until no crystals remain

Pour the solution into a watering can fitted with a rose, or use a pump sprayer. Choose a dry day with no rain expected and as little wind as possible. That way, the mixture lands where it’s meant to: directly onto the moss and into the joints.

When watered or sprayed precisely, the green growth often turns yellowish within the next one to two days and then goes brown. The plants break up more easily, and a firm sweep is usually enough to dislodge them. Stubborn patches can be treated again with a second, targeted application.

Keeping the soil healthy and the garden undamaged

As useful as this mineral salt is, excessive amounts can put strain on the soil - particularly if large areas are watered regularly. A few basic rules help prevent harm in the garden.

Area	Recommendation
Patios & paved paths	Suitable if used sparingly and applied precisely into the joints
Flower beds, vegetable plots	Best avoided, as the roots of cultivated plants can be damaged
Lawn edges	Use only in stone or slab joints; don’t allow it to run into the grass
Frequency	Once or twice a year is usually more than enough

If you live on a slope, or you have areas where water drains strongly towards beds, keep a close eye on where run-off goes. If a concentrated solution repeatedly reaches a vegetable patch, it can affect the soil life there.

Common mistakes that reduce effectiveness

In practice, the same missteps crop up again and again, and they tend to make the result weaker than it could be. Three typical examples:

• Applying too thickly: piling it up in centimetre-deep heaps wastes product and is more likely to burden the soil, without giving noticeably better results.
• Rinsing immediately: flushing with lots of water straight away often washes the salt out of the joint before it has had time to work.
• Treating before heavy rain: if a downpour is due soon after application, the solution spreads unpredictably and becomes heavily diluted.

A calm, relatively dry winter day - plus a little patience - is the better option. The effect builds over hours and days, not in minutes.

How to combine the method with other practical measures

Bicarbonate of soda isn’t a replacement for good joint maintenance, but it can make it much easier. If you also use mechanical methods in spring or summer, hard surfaces stay cleaner for longer. A few useful ideas:

• Re-sand joints annually, ideally using weed-suppressing jointing material.
• When laying new paths, plan for joints that are as narrow as possible from the outset.
• Arrange drainage so persistent damp patches don’t form.

Used this way, many areas only need occasional treatment. In shady spots, where moss naturally thrives, the combination of good drainage, periodic sweeping and a targeted winter treatment is particularly helpful.

Risks, limits and sensible alternatives

If you have pets, remember that larger amounts of freshly applied powder can cause stomach upset if dogs or cats lick it. Once it has been brushed into the joints and has taken on moisture for a few hours, the risk drops significantly - but it still pays to keep an eye out.

With very old natural stone slabs or delicate finishes, test a small patch first. In rare cases, residues can leave slight discolouration around the joints, especially where other salts or fertilisers have already been introduced.

If you’d rather avoid kitchen-based products altogether, you can stick to the traditional approach with a joint scraper, wire brush or paving joint brush. Chemical-free hot-water machines are also used, although they tend to be noisier and consume a lot of energy.

Ultimately, the appeal of a winter bicarbonate of soda treatment is its simple shift in timing: you move the main effort away from busy spring weeks into the quieter, cooler season - and let an unassuming white powder do much of the work while it sits in the joints.