How Blockade Works: Stopping Weeds Before They Start
How Blockade Works: Stopping Weeds Before They Start
Does It Really Stop Seed Germination?
When people hear that NZLA Blockade “stops seeds from germinating,” it sounds simple enough. In reality, it isn’t quite that literal. The product doesn’t magically prevent a seed from waking up. Instead, it creates a treated layer in the soil that prevents new seedlings from establishing.
The Science Behind It
Blockade (the same active used in Syngenta’s Barricade) contains 480 g/L Prodiamine, a pre‑emergent herbicide that inhibits cell division in the root tips of newly germinating weed seeds. That means it works in the soil before weeds break the surface. Once applied and watered in, it binds to the upper layer of soil and stops the tiny roots and shoots of germinating weed seeds from developing properly. Without that early growth, the seed never becomes a weed in your lawn or garden bed.
Why We Say “Stops Germination”
The phrase is a handy shortcut that even we use at NZLA. Homeowners aren’t usually interested in the fine detail of root inhibition and biochemical processes. Saying it “stops weeds from germinating” gets the point across quickly: use Blockade before weeds appear and they won’t show up.
Blockade Won’t Kill Existing Weeds
One thing to remember is that Blockade is strictly a pre-emergent herbicide. It won’t do anything to weeds that are already up and growing. If you’ve got broadleaf weeds or grasses showing in your lawn or garden beds, those need to be controlled separately with the right post-emergent spray or by hand removal. Blockade’s role is prevention, creating a barrier that stops new weeds from establishing after the old ones are dealt with.
If a weed has already sprouted and pushed through the soil surface, Blockade won’t touch it. The herbicide only works on seeds at the point of germination, stopping roots and shoots from establishing. Once the weed is visible, it’s past that stage and will keep growing as normal. In that case, you’ll need a post-emergent herbicide or manual removal to deal with what’s already there. Blockade then takes over by preventing new weeds from appearing, so you don’t fight the same battle again in a few weeks.
Where You Can Use It
Lawns: Safe on established turf such as Kikuyu, Couch, and Ryegrass.
Garden Beds: Can be used around ornamentals to keep beds weed-free for months.
Timing: Best applied in spring or autumn when soil is warm and weeds are about to germinate.
Application Rates
The application rate for Blockade depends on where you are using it. On established turf, the rates are lower, with 30 ml per 100 m² being the upper end of the label range. In garden beds and ornamental areas, the rate is higher and can go up to 80 ml per 100 m². This is because bare soil and garden settings generally need stronger protection, as there is no turf cover to compete with weeds. Always follow the label rates for the area you are treating, and measure carefully to avoid under- or over-applying.
Is Blockade Safe on Cool‑Season Grasses Like Ryegrass and Fine Fescue?
In New Zealand, Blockade is only registered for use on warm-season grasses such as Kikuyu and Couch. That’s what the label allows and what the product has been tested for locally. Overseas, particularly in the United States, the same active ingredient is also registered for cool-season grasses like Ryegrass and Fine Fescue. Some people here may choose to apply it on those grasses, but this would be considered off-label use in New Zealand. If you’re thinking about trying it, understand that it’s at your own discretion and not part of the official label guidance.
How Long It Lasts
Blockade provides long-lasting control, with protection for up to six months. That’s why timing the application is so important: put it down before the weeds move, and you’ll stay ahead of them for the season.
When to Apply Blockade
The best time to use Blockade is just before weeds start germinating. In New Zealand conditions, that usually means applying it in early to mid-spring as soil temperatures rise, or again in autumn before winter annuals emerge. The key is to have it in the soil before weed seeds wake up, because once weeds have already sprouted, Blockade won’t control them. By treating at the change of seasons, you build that protective barrier in advance and keep your lawn or garden clean for months ahead.
The Importance of Watering In
Watering Blockade in after application is critical for it to work. The product needs to move from the leaf surface down into the top few centimetres of soil where weed seeds germinate. If it sits on the surface, it won’t form the protective barrier and you risk losing effectiveness. Irrigation of 6–10 mm within 7 days of applying is required to activate Blockade and lock it into the soil, creating that long-lasting shield against weeds.
While Blockade needs water to move into the soil, relying on rainfall isn’t the best approach. Showers can be unpredictable and heavy rain increases the risk of the product moving unevenly across the surface. To get consistent results, it’s better to control the process yourself with irrigation or a hose. That way you know the herbicide has been watered in properly and is bound into the soil where it needs to be, without the risk of runoff or patchy coverage.
Using Blockade in Garden Beds
For Blockade to work properly in garden beds, it needs to bind to the soil surface. If there’s a layer of mulch in the way, the herbicide will sit on top and won’t form the protective barrier where the weeds are actually germinating. Before applying, pull mulch back so you’re treating the soil itself. Once the product has been applied and watered in, you can replace the mulch on top without affecting the effectiveness.
Mixing Blockade with Glyphosate for Paths and Driveways
Blockade can be tank-mixed with glyphosate to provide both immediate and long-term weed control in non-turf areas like garden paths and gravel driveways. In this situation, the glyphosate acts as a knock-down herbicide, killing the existing weeds, while Blockade creates a residual barrier in the soil to prevent new ones from germinating. After spraying, it’s important to leave the application undisturbed for the required time so the glyphosate can move through the foliage and do its job, typically 24 hours depending on conditions. Once the knock-down phase has taken effect, follow up with irrigation to wash the Blockade into the soil profile, ensuring it’s activated and ready to provide months of ongoing protection.
Don’t Use Blockade Where You Need to Seed
Blockade forms a barrier in the soil that prevents seedling roots and shoots from developing. That’s great news for stopping weeds, but it also means it will stop grass seed from germinating too. If you’re planning to sow a new lawn or patch bare areas, Blockade is not the product to use. Putting it down before seeding will dramatically reduce your germination success and leave you with thin or failed establishment. Always get your new lawn up and growing first, then wait until it’s well established before applying Blockade.
When Can You Use Blockade on a New Lawn?
You’ll need to wait until your new lawn is fully established before using Blockade. As a rule of thumb, hold off until you’ve mown the lawn at least three times and the grass has developed a strong, even cover. At that point, the turf is hardy enough to handle the barrier effect in the soil. Applying it any earlier can interfere with seedling development and leave you with weak or patchy areas. Patience here pays off, let the grass settle in before locking in long-term weed protection.
Using Blockade Around Pets
Blockade is safe to use in lawns and garden beds when applied according to the label, but a bit of care is still needed with pets. Keep dogs and cats off treated areas until the spray has fully dried and the product has been watered in. This prevents them from walking through wet spray or ingesting residues while grooming. Once the lawn or soil surface is dry and settled, pets can return as normal without risk. For added peace of mind, many people choose to apply in the evening and water in straight away, so the area is safe again by morning.
