Broadleaf weeds are easiest to kill in spring when they are young. A dandelion at two to four leaves takes up selective herbicide readily, translocates it through leaf tissue, and dies before setting seed. Wait until rosettes are fully formed and you are spraying harder, at higher rates, for worse results.

The window depends on two things arriving together: weeds that are actively growing, and grass that is doing the same.

Why both plants need to be in active growth

Selective herbicides work by entering the weed through leaf tissue and travelling through the vascular system. That process, called translocation, only happens properly when the plant is growing. Apply to a stressed or semi-dormant weed and the active ingredient sits on the surface rather than moving through it.

Grass that is stressed is also more vulnerable to phytotoxicity (herbicide damage to non-target plants). Soil temperature at 10 cm depth should generally be above 10 degrees C before a spring application. The Met Office publishes soil temperature data by region, which is more useful than picking a calendar date.

In most parts of the UK that window opens somewhere between mid-March and early April, depending on site location and how hard the winter was. It closes again if a dry spell stresses the sward before summer.

The label is a legal document, not a suggestion

Every plant protection product used on professional or amenity land in the UK must hold a current HSE authorisation, shown as a MAPP number on the label. Using the product above the stated rate, off-label, or for a purpose not listed is an offence under the Plant Protection Products Regulations 2011. The rate on the label is the rate at which the product was tested and authorised. Going above it does not improve control on young weeds.

Before each application, check:

  • Minimum and maximum application rate (litres or grams per hectare)
  • Target weed species and the growth stage at which to treat
  • Minimum air temperature and any wind speed restriction
  • Rain-free and frost-free periods required before and after spraying
  • Re-entry interval for publicly accessed areas

Rain exclusion periods, typically six to eight hours, allow the active to bind to leaf tissue and begin absorbing. Spraying when rain is forecast within that window is ineffective and carries run-off risk.

Matching the product to the weed

Annual broadleafs such as chickweed are most vulnerable as seedlings. Perennials, including dandelion and ribwort plantain, respond best before flower stalks form, when energy is still in vegetative growth. Clover and speedwell are harder targets and often need a product specifically listed for those species and good spray coverage. Some require a follow-up application at the minimum interval stated on the label.

Do not spray during drought. Stressed weeds do not translocate well, and stressed grass is at greater risk from phytotoxicity.

Record-keeping after treatment

Pesticide use on professional land in the UK must be logged. Each record should include the product name, MAPP number, rate applied, area treated, date, weather conditions, and operator details. Three years is the standard minimum retention period, and most grounds maintenance contracts require records as part of an agrochemical audit.

Wait at least three to four days before mowing the treated area, or longer if the label specifies it. Cut too soon and you remove the spray deposit before the weed has taken up enough active to make the treatment count.

Did you know? Dandelion seeds can remain viable in soil for several years. Treating plants before they flower in spring prevents that season's seed being added to the seedbank, but existing seed means good spring timing needs to become a regular annual habit in heavily infested turf.

Frequently asked questions

What temperature is too cold to spray selective weed killer on turf?

Most selective herbicides recommend a minimum air temperature of around 10 degrees C and similar soil temperature for effective translocation. Applying below this threshold reduces efficacy and can increase the risk of turf scorch. Check your specific product label, as minimum temperatures vary by active ingredient.

Can I use selective herbicide on newly seeded turf?

Most selective broadleaf herbicides advise waiting until a new lawn or reseeded area has been cut at least two or three times before treatment. The exact recommendation varies by product; always check the label for the minimum establishment period. Treating too early risks damaging the young grass.

How long after spraying weed killer should I wait before mowing?

Most products recommend a minimum of three to four days before mowing, to allow the herbicide time to enter and translocate through weed tissue. Some products specify a longer interval; always follow the label instruction rather than assuming a standard rest period applies.

Do I need to keep records of weed killer use in amenity areas?

Yes. UK law requires that pesticide use on professional or commercial land is logged, including the product name, MAPP number, rate applied, area treated, date, weather conditions, and operator details. Records should generally be kept for at least three years.

Why is selective weed killer not killing the clover in my turf?

Clover can resist some selective herbicides that work well on dandelions and plantains. You need a product specifically labelled for clover control, often containing MCPA, fluroxypyr, or clopyralid. Coverage also matters because clover leaves are waxy and spray can bead off. A follow-up application at the label interval is often needed for reliable control.