What time of day should you spray weed killer?

What the trials actually found

Two studies have put a number on this properly, both published in the journal Weed Technology, and between them they are worth more than every gardening website on the subject put together. Waltz and colleagues at the University of Nebraska ran field trials on velvetleaf across two seasons. In the first, glyphosate applied before sunrise gave 54 per cent control while the same treatment after midday gave 100 per cent. In the second, before sunrise gave 69 per cent, midday again gave 100 per cent, and after sunset dropped to 37 per cent. Same product, same rate, same field. The only variable was the hour on the clock.

Norsworthy and colleagues had already shown why. They sprayed at six in the morning, eleven, four in the afternoon and nine at night, across three weed species, and measured how much spray the leaves actually caught. Night applications intercepted less: 17 per cent less on one species, 67 per cent less on another, 70 per cent less on the third. The cause is diurnal leaf movement. Plenty of plants change the angle of their leaves through the day and fold them up towards evening, so a leaf that presents a flat target at noon presents an edge at nine at night, and a good part of your spray goes past it onto the ground.

Be careful how far you carry those percentages, because I am not going to pretend they are British garden data. Velvetleaf, prickly sida, hemp sesbania and sicklepod are American arable weeds, not the couch grass and willowherb coming up through a drive. What travels is the mechanism, which is physical and applies to any plant that moves its leaves, and the size of the effect, which is large enough that the caveats do not rescue a dusk application. Nobody has run the equivalent trial on a British driveway, and if a website quotes you a UK percentage for this, ask them where it came from.

Wet or dry leaves: the honest answer rather than the confident one

Every gardening website tells you flatly never to spray onto wet grass or a dewy weed. Only one UK label I have read says anything about it at all, and what it says is more precise than the blanket rule. Roundup ProActive (MAPP 21225), the professional 360 g/L glyphosate, states that a covering of dew may reduce efficacy where run-off occurs. Read the conditional in that sentence, because it is doing the work. Not dew reduces efficacy. Dew may reduce efficacy where run-off occurs.

The spray application research says the same thing, and it is genuinely less settled than the confident advice suggests. Trials found that larger droplets ran off dew-saturated foliage noticeably more than small ones, and yet the efficacy of the spray was not altered by droplet size. In other words the visible runoff and the actual result did not line up as neatly as you would expect. What that leaves you with is a practical test rather than a rule: if the leaves are beading and the water is running off them, your spray runs off too and you are wasting it. If there is a light film that is already lifting, you are fine. Heavy dew is the problem, not moisture as such.

Same logic on a hard surface, with one extra thing to watch. Water on block paving or concrete does not sit in a thin film the way it does on a leaf, it collects in the hollows and along the joints, which is exactly where the weeds are. Spray into a wet joint and you have diluted the dose at the only place it needed to be full strength. Wait for the surface to go matt and then go.

The real reason to avoid dawn and dusk has nothing to do with dew

Early morning and evening are exactly when a temperature inversion sits over the ground: a layer of cold, still air trapped underneath warmer air, with almost no mixing going on. The Scottish Government's Farm Advisory Service puts it plainly for anyone spraying in the UK, that inversions are more common early in the morning and in the evening and should be avoided if possible.

In an inversion the fine end of your spray does not settle and it does not disperse. It hangs there in suspension and then drifts off sideways in whatever slow movement the air has, which in a garden means your neighbour's border, your own lawn, or the bed of shrubs at the end of the drive. It is the classic cause of damage that turns up somewhere you never pointed the sprayer, days later, with no obvious explanation.

Which brings the two halves of this question together. The way to spot an inversion without any instruments is that if you can see fog, dew or frost, you are already in one. So the dew on the grass at seven in the morning is not really the problem. It is the visible marker of the actual problem, which is the still trapped air it formed in. That is why the two halves of this question, the dew and the hour, are the same question wearing different clothes. Guidance on inversions puts them at their strongest around sunrise and dissipating roughly two hours after it, and warns that sunset is a poor cue in the evening because the inversion can set in hours before the sun goes down.

How still is too still

The instinct is to wait for a dead calm day, and that instinct is wrong. The Farm Advisory Service gives the numbers: ideally spraying is done in cool, humid conditions with the wind between 2 and 4 mph, and the optimum window runs from 1.2 to 6 mph. There is a floor as well as a ceiling, and the floor exists because dead calm and an inversion are the same weather.

Two to four miles an hour is a breeze you can just feel on the back of your hand and see in the tips of long grass, nothing more. It is enough for the air to be mixing, so what you spray comes down where you aimed it, and it is little enough that a fine mist is not being carried off the target. Above about six you can watch your own spray pattern bending, and that is the point to stop, because on a drive the plants you would rather keep are usually a metre away at most.

So which hour, in practice

Mid-morning, once the dew has properly gone and the air has started moving, or the middle of the afternoon. That gives you the weeds awake and their leaves flat to the sky, the inversion broken up, a bit of breeze to keep the mist honest, and enough daylight left to see what you have covered. On a hot bright day in high summer the middle of the afternoon is the one to be careful with, not because of the hour but because a weed that has shut down in a drought takes very little up, which is the same reason the labels tell you not to treat plants whose growth is impaired.

The two hours to skip are the first couple after sunrise and the last couple before dark. You lose on interception because the leaves have moved, you lose on the dew if it is heavy, and you take on a drift risk you cannot see happening. None of that is worth the convenience of doing it before work.

Two things sit outside this answer, so you know where to look. Which months to spray in is a different question with a different answer, and it sits on the answer about the best time of year to spray weeds. How long a gap you need before rain is a third one, and the labels are specific enough about it to deserve their own page, which is the answer on spraying weeds when it is raining. Get all three right on the same afternoon and the root dies. Get the hour wrong and you have paid full price for a third of a job.