Tank mixing lawn products: when it's allowed and how to do it

If you’re spraying a selective herbicide on the lawn this weekend and a liquid fertiliser next weekend, it’s tempting to put them in the same tank and do both at once. Sometimes that’s fine. Often it isn’t, and the consequences range from a blocked nozzle to a damaged lawn or a weed treatment that doesn’t work.

The rule is simple: only mix products together when both labels allow it. This guide explains how to find that information, what to do when the labels are silent, and how to mix safely when a combination is allowed.

What can go wrong

Products can be incompatible in several ways.

• Physically. The mix separates into layers, curdles, forms sludge or sediment, or foams. It blocks nozzles and filters, and the product doesn’t go on evenly.
• Chemically. One product changes how the other works. A herbicide may control fewer weeds, or a fertiliser may change how the lawn reacts to a herbicide.
• On the lawn. A mix can stress or damage grass that would tolerate each product applied on its own. Buffalo, which is already sensitive to some herbicides, is a particular concern.
• On timing. Each product has its own directions on weather, mowing, watering in and re-entry. If those directions conflict, one product will be applied the wrong way.

The Grains Research and Development Corporation, writing for farmers, puts it plainly: correct mixing order reduces the risk of products interacting in a way that reduces their efficacy or the stability of the mix, and bringing concentrated products into contact with each other usually results in undesirable interactions.

Read both labels

The APVMA’s labelling code allows labels to include statements about tank mixes and compatibility, and requires specific incompatibilities to be stated in the label’s general instructions. So the label is where you look.

Check each label for:

• a compatibility or tank-mix section, which may list products or product types it can be mixed with;
• an incompatibility statement, which lists what it must not be mixed with;
• a mixing order, if a mix is allowed;
• directions that conflict with the other product’s, such as one needing watering in straight away and the other needing to stay on the leaves.

Then decide:

Home garden products often don’t mention tank mixing at all. That’s not permission. If you can’t find the information, apply each product on its own, as its label directs. You can read the approved label for most registered products on the APVMA’s PubCRIS database, which sometimes has more detail than the pack.

Combined products

Some products already combine two jobs, such as a fertiliser with a herbicide (often called weed and feed), or a wetting agent with fertiliser. These have been formulated and registered as one product, so you follow that product’s single label. They’re a better choice than making your own mix if you want both jobs done at once, provided the label lists your grass and the weeds you have.

The jar test

When the labels allow a mix, a jar test checks whether the products stay mixed in your water before you commit a whole tank. It doesn’t tell you whether the mix is safe for your lawn or will work; only the labels do that. It just checks the physical mix.

GRDC’s fact sheet on spray mixing describes the method for farm sprayers. Scaled down for a home lawn, it looks like this:

1. Use a clear glass jar with a lid, kept for this job and labelled.
2. Use water from the same source you’ll spray with: the same tap or tank. Water quality affects mixing.
3. Work out small amounts in the same proportions as the full mix. If your tank mix is one sprayer-full, the jar test might be a twentieth of it. Use syringes or a small measuring cylinder kept only for chemicals.
4. Partly fill the jar with water, in the same proportion as you’ll partly fill the tank.
5. Add each product in the order the labels give, one at a time, mixing after each one. Let dry products dissolve fully before adding the next.
6. Top up with water, seal the lid and shake to mix.
7. Let it stand for at least five minutes, and ideally up to 30 minutes.
8. Look at it.

Reading the result

• Even and consistent throughout: the products appear physically compatible in your water.
• Layers or bands that mix back in with a shake and stay mixed for a while: the mix may only work with constant agitation. A pump-up sprayer doesn’t agitate, so think twice.
• Layers that return quickly, sludge, flakes or sediment on the bottom: don’t use this mix.
• A creamy layer on top: the products aren’t staying mixed. Don’t use it.

Wear gloves and eye protection for the jar test, as you would when mixing. Dispose of the contents as you would leftover spray mix, never down a sink or drain.

Mixing order

Follow the mixing order the labels give. GRDC’s general principles for farm spraying are:

• Start with clean equipment and a tank partly filled with clean water.
• Add products one at a time, and mix each through before adding the next.
• Add products according to their formulation. GRDC’s basic order starts with water conditioners, then dry products such as water-dispersible granules and dry flowables, then suspension concentrates, then emulsifiable concentrates, then soluble liquids, with oils always last. Surfactants go in near the end to limit foaming.
• Never combine products in concentrated form, such as pouring one into the other’s measuring cylinder, or adding two products to an almost empty tank together.

If the labels don’t give a mixing order, don’t guess. Apply the products separately.

GRDC also notes that different brands of the same chemical can behave differently in a mix, because each formulation contains different additives. A mix that worked with one brand may not work with another.

How to mix a tank

When the labels allow it and the jar test looks good:

1. Calibrate your sprayer with clean water so you know how much mix you need for your area. See the sprayer calibration guide.
2. Work out each product’s amount with the Product Application Calculator, using each label’s rate for your area.
3. Put on the protective equipment both labels list. If they list different equipment, wear all of it.
4. Mix outdoors on a hard surface, away from drains, children, pets and garden beds.
5. Partly fill the sprayer with clean water, as the labels direct.
6. Add the first product, close the sprayer and swirl or shake gently to mix.
7. Add the next product in the order the labels give, and mix again.
8. Top up with water to the volume you need, and mix.
9. Use it straight away. Pump-up sprayers don’t agitate, so shake the sprayer gently from time to time while you work, if the labels suggest it.

Rinse each measuring cylinder between products, and add the rinse water to the tank.

Applying the mix

A tank mix must be applied in a way that suits both labels.

• Weather: use the stricter of the two labels’ conditions on wind, temperature and rain.
• Mowing: follow the longer of the two labels’ mowing directions.
• Watering in: if the labels agree, follow them. If they don’t, you shouldn’t be mixing these products.
• Re-entry: keep people and pets off the lawn for the longer of the two labels’ re-entry periods.
• Edges: home garden herbicide labels warn against letting spray contact or drift onto plants you don’t want killed. A fertiliser in the mix doesn’t change that.

Cleaning up

• Spray out the whole mix on the area you planned. Don’t keep it for another day.
• If you have mix left over, apply it to the treated area at the label rate if the labels allow it. Otherwise, take it to a household chemical collection. Never pour it down a sink, toilet or drain.
• Clean the sprayer thoroughly as the labels direct. A tank-mix residue can affect the next product you use in it.
• Keep one sprayer for herbicides and label it, so traces of weed killer never reach plants you want to keep.

A simpler approach

For most home lawns, applying products separately is easier and safer. A selective herbicide this weekend and a liquid fertiliser next weekend means each one goes on in the conditions its label intends, and if something goes wrong, you know which product caused it.

If you want to cut down on separate jobs, look for a single registered product that does both, or plan your products so each application fits the season.

Common mistakes

• Assuming a mix is fine because a label doesn’t say otherwise.
• Pouring one concentrate into another before adding water.
• Adding products in the wrong order, or all at once.
• Treating a jar test as proof the mix is safe for the lawn.
• Swapping brands in a mix the label allows for named products.
• Keeping leftover mix for another day.
• Following the weaker of two labels’ directions on weather, mowing or re-entry.

Log it

Log a tank mix as one application in your Lawn Journal, listing every product, the amount of each, the area and the date. If anything unexpected happens, such as yellowing or poor weed control, you’ll have the details to check against the labels.