A homeowner named John asked about weed control between retaining wall bricks and in planters, plus cockroach and earwig infestations in a planter. A nursery recommended "KILLZALL or Over-the-Top" herbicides.
Weed Control in Hardscapes (Retaining Walls, Pavers)
Pre-Emergent Strategy (Prevention)
- Polymeric sand: Sweep into joints; hardens with moisture to block germination
- Pre-emergent herbicides: Pendimethalin, prodiamine, or dithiopyr applied early spring
- Landscape fabric: Under new installations; not retrofit-friendly
Post-Emergent Options
- Glyphosate (Roundup, Killzall): Non-selective systemic; kills roots; 2-3 day biodegradation
- Glufosinate (Finale): Contact action; faster burndown; less translocation
- Organic options: Citric acid/clove oil (BurnOut), acetic acid (20% vinegar), iron chelate (Fiesta)
Application Best Practices
- Shield desirable plants with cardboard or plastic
- Apply on calm, dry days (no rain for 6+ hours for glyphosate)
- Use shielded sprayer or brush-on for precision
- Re-treat every 4-6 weeks during growing season
- Newspaper/mulch: 3-4 layers wet newspaper + 2-3" mulch in planters
Planter Pest Management
Cockroaches
- Not typical garden pests—usually indicate moisture/ sanitation issues
- Bait stations (gel or container): Most effective; place in corners, under lips
- Reduce moisture: Improve drainage; water at soil level, not foliage
- Remove debris: Decaying matter, stacked pots, stored materials
- Boric acid dust: In voids, under planters (keep away from pets)
Earwigs
- Generally beneficial—eat aphids, mites, decaying matter
- Become pests only at high populations damaging seedlings/flowers
- Traps: Rolled newspaper, tuna cans with oil, corrugated cardboard
- Diatomaceous earth: Around planter base
- Reduce harborage: Limit thick mulch, dense groundcovers near planters
Planter Soil Management
The original responder suspected overwatering. Key practices:
- Well-draining mix: 1/3 compost, 1/3 potting soil, 1/3 perlite/pumice
- Water deeply but infrequently: Allow top 1-2" to dry
- Elevate planters: Pot feet or risers for airflow
- Annual soil refresh: Replace top 1/3 or full repot
Herbicide Selection Clarification
- "Killzall" = glyphosate (non-selective, systemic)
- "Over-the-Top" = typically sethoxydim or fluazifop (grass-selective)
- For hardscapes: Glyphosate is appropriate
- For planters: Avoid herbicides—hand-weed or use mulch
Integrated Approach
- Clean out planter; inspect for roach harborage
- Repot with fresh, well-draining mix
- Apply gel bait for roaches in discreet locations
- Hand-weed hardscapes; apply polymeric sand to joints
- Spot-treat persistent weeds with glyphosate using shielded sprayer
- Mulch planters with 1-2" coarse bark
Regular monitoring replaces reactive spraying—weekly walk-through catches problems early.
