Weeds Growing Through The Joints

What's happening?

Weeds in paving joints do not come up from below. On a properly built pavement the sub-base is graded stone with almost nothing for a seed to root in; what the weeds are growing in is the top of the joint, where windblown dust, leaf litter, moss and worn jointing sand have built up into a thin, damp seed bed. That is why weeding is endless: pulling the plant out leaves the growing medium behind. The other reason joints keep filling is that the sand in them has gone. Kiln-dried sand is what locks blocks together, and once it washes out (usually into a low spot, down a gully or out from under an open edge) the joint becomes an open gutter that collects everything.

What usually causes it?

• Jointing sand washed out by rain, hoses or a pressure washer, leaving open joints that trap debris
• Organic matter building up in the joints from leaves, moss, grass clippings and airborne dust
• Overhanging trees and hedges dropping seed and leaf litter straight onto the paving
• Joints staying permanently damp because the surface has no fall or sits in shade
• Nearby borders, lawn edges and gravel beds seeding into the paving
• Deep-rooted perennial weeds such as bindweed or horsetail spreading in from adjoining ground
• No edge restraint, so joints at the perimeter open up and sand escapes sideways

Is it dangerous?

Weeds are a nuisance rather than a hazard, though moss and algae in wet joints make a surface slippery and a grassy joint hides an uneven edge. The real cost is structural. A block pavement gets its strength from the interlock between blocks, and that interlock depends on full sand joints. Empty joints let blocks rock, spread and wear at the arrises, and let water down to the bedding layer. Treating weeds without refilling the joints treats the symptom.

Can I fix it myself?

Clear the joints out properly, then close them. Scrape the organic build-up out with a joint knife or a stiff brush, remove the debris rather than sweeping it back in, and let the surface dry. Refill block paving joints with kiln-dried sand, brushing it in and repeating after rain until they stay full: this is the step most people skip. Cut back anything overhanging that drops leaves and seed. For perennial weeds, identify what you have before treating: horsetail, bindweed and Japanese knotweed all need a different approach, and knotweed near a driveway is a matter to identify rather than spray. Use only products approved for domestic use on paths and drives, and follow the label.

When should you call a driveway and patio installer?

Call a driveway or patio specialist when the joints will not hold sand, when blocks have started to move or spread, or when you have identified a persistent perennial weed spreading under the paving. Repeated weeding is a sign the joints have failed rather than that you are unlucky. If you suspect Japanese knotweed anywhere near the drive, get it identified first, because it is treated as controlled waste and the paving work follows the treatment plan rather than the other way round.

What will the driveway and patio installer actually do?

• Clear the joints of organic matter mechanically rather than washing it deeper into the pavement
• Check whether jointing sand is present to depth, and identify why it has been escaping
• Reinstate or repair the edge restraint so the perimeter joints stay closed
• Re-sand the joints and vibrate the surface where appropriate so the blocks lock together again
• Replace a failed joint with a stabilising or polymeric jointing product where the pavement suits one
• Correct low spots and falls where standing water is stripping the joints in the same place every time

Who fixes this

• Patio cleaning and resealing

Questions people ask

Sources & References

1. RHS: Weeds and weed control
2. RHS: Japanese knotweed
3. GOV.UK: Prevent Japanese knotweed from spreading
4. Interpave: Maintenance of concrete block permeable paving