One of the most frustrating things after laying a new patio or block paving is when tiny weeds start to pop up between the cracks. After the hard work and expense of a garden project, you want it to look great for years.
The most effective way to prevent weeds is through proper installation from the start. Here are the steps to properly prepare the site, apply preventative measures, and safely get rid of existing weeds so they don't return.
1. Proper Preparation and Base
One of the biggest mistakes DIYers make is not digging deep enough or using the wrong base materials, such as garden soil. Soil shifts over time, causing the paving to sink, move, and crack, creating openings where weeds will quickly take root.
Digging: Excavate around 15cm (about half a foot) of soil to allow room for the base layers and the thickness of the paving slabs themselves.
Sub-base: Add around 10cm (4 inches) of hardcore or gravel. This creates a sturdy foundation and allows for proper drainage during wet British weather.
Underlayment: Top the hardcore with 2.5cm to 5cm (1 to 2 inches) of limestone grit or sharp sand. This creates a smooth, level bed for the stones, facilitates further drainage, and deters weed growth and insects due to its alkaline properties. Always ensure the sub-base is properly compacted before laying paving to prevent sinking.
2. Laying the Paving
Use the "click and drop" method to avoid disturbing the sand bed. Push a slab flush against the adjoining one, and when it "clicks," drop it into place.
If you are using perfectly square slabs, use paving spacers. Avoid placing them directly against one another, as this pushes them out of alignment and prevents you from filling the joints properly with sand to stop weeds. Avoid long continuous straight lines where possible; opt for paving patterns that create a stronger, interlocking bond that is less likely to shift.
3. Filling Joints with Sand
Once laid, sweep jointing sand into the gaps to eliminate spaces for weeds.
Polymeric Sand (Block Paving Jointing Sand): When activated by water, the strong polymer bonding agents create a hard, gel-like barrier that makes it difficult for weeds to grow through from underneath or take root from the top. It typically lasts up to 10 years. Tap the paving slabs with a broom handle to vibrate the sand deep into the joints, continuing until the sand sits roughly 3mm below the top of the paving. Follow the manufacturer's washing instructions carefully to prevent a permanent haze from forming on the surface of your slabs.
Kiln-Dried or Regular Sand: An alternative to polymeric sand. It is easier to apply and carries no risk of chemical haze, but it requires topping up every few years as wind and heavy rain will naturally erode it.
4. Applying a Sealant
Sealing your patio adds another layer of protection. Sealants harden the jointing sand and create a barrier that encourages rainwater to run off rather than seeping into the joints, which washes away airborne weed seeds rather than absorbing them. It also protects porous concrete slabs from general staining. Note: Always clean the paving thoroughly beforehand, and if using polymeric sand, most manufacturers recommend waiting at least 90 days before applying a sealant.
5. Managing Existing Weeds
Even well-laid patios will eventually see some weed growth, usually from seeds blown in by the wind or washed over during rain.
Hand-Pulling: The most environmentally friendly option. Always wear gardening gloves and ensure you pull out the entire taproot (crucial for weeds like dandelions). Be careful not to disturb seed heads on the flowers, as this is how they spread.
Jet Washing: A pressure washer can blast away shallow-rooted weeds, purslane, and moss. Use a gentle setting to avoid blasting out the jointing sand or damaging the slabs. Always fill in any gaps with fresh jointing sand once the area is dry.
Natural Weed Solutions: Avoid boiling water (it rarely works and can damage the surface of the stone) and vinegar (which is unproven and can stain certain masonry). Instead, look for environmentally friendly, pet-safe herbicidal soaps that kill weeds on contact. Ensure the product is safe for use on concrete and masonry before applying.
