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Q: What's the shortest wait time for spreading pre-emergent AFTER putting down new lawn seed (patching)?

Specifically, I'm using 1) Scotts Lawn Soil (contains a fertilizer) 2) Pennington Smart Seed Sun & Shade

I'm asking this because I'd like to patch areas of the lawn myself when the nightly lows are ABOVE freezing temps in New Jersey, which will be "1 Week" before the lawn care company will spread their pre-emergent on April 1st. (Long story, but I cannot call them to delay their pre-emergent application)

Edit: From past years, it appears that the lawn company uses a pellet-type pre-emergent, such as "Preen"

Thanks, CG

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  • Its difficult to answer without knowing the exact pre-emergent the lawn care company is likely to use - pre emergents vary in formulation depending on what plants/grasses they're targeting.
    – Bamboo
    Mar 13, 2019 at 22:17
  • You can do it the same day, but will it work? doubtful. What is a weed, I only know about cover crops?
    – black thumb
    Mar 13, 2019 at 22:31
  • Bamboo- Thanks, good point. My best guess years is that the lawn company might be using "Preen" (otherwise "Weed & Feed") since I've noticed they've used a pellet-type pre-emergent in past years.
    – user25127
    Mar 13, 2019 at 22:53
  • There are 4 different Preen products for use on lawns... one specifically to prevent crabgrass, so still not sure which one they will use,but as I understand it, pre emergents are fine to use when your lawn seed has grown to about 3 inches high (roughly) but not prior to that. So a week certainly isn't long enough...Most will remain present in the soil for a full growing season too, unfortunately.Suggest you talk to your lawn care company....
    – Bamboo
    Mar 13, 2019 at 23:12

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Grass seed won't germinate if the soil temperature is below about 10C (50F) which usually means daytime temperatures of 15 to 20C (60 to 70F).

Sowing seed when the nightly lows are "just" above freezing will probably be a waste of time. Even if it germinates, grass doesn't grow when the overnight temperature is below 5C (40F)

I'm in the UK, so I have no idea what "pre-emergent" means - though I guess some combination of fertilizer and weedkiller.

Weedkiller is not going to do ungerminated seed, or newly germinated grass, any good. I would wait at least 4 to 6 weeks after germination before using any weedkiller. One (apparently US) website says "A standard pre-emergent herbicide should not be applied until at least three to four months after seeding the area." However products with the active ingredient Siduron (also called Tupersan) can be used during seeding.

If you really can't cancel what the lawn care company is going to do, you will probably be better off reseeding the lawn in September, not now.

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  • pre emergents aren't available in the UK - but they are herbicides which interfere with root formation, often used to deter things like crabgrass in lawns in the States...
    – Bamboo
    Mar 13, 2019 at 22:18
  • @Bamboo: but there are in UK other kind of pre-emergent weedkillers, which are totally different of what you are describing. On fields after you seed, you can apply some pre-em to kill weeds before the crop will grow. Same name, contrary meaning (contrary point of view: one about emerging weeds, one about emerging crop).
    – Giacomo Catenazzi
    Mar 14, 2019 at 7:04
  • @GiacomoCatenazzi - pre emergents are not available for use by amateur gardeners/horticulturalists, but they may well be available for agricultural users such as farmers and maybe groundsmen (golf greens, sports fields, though I'm not certain whether they have access to them) within the UK - since our rules are currently EU rules, I imagine the same rules apply within the EU. I am not sure what you mean by 'different pre-ems', unless you mean the widely practised spraying of crops with glyphosate a few days before cropping to kill off any weeds and stubble left behind afterwards.
    – Bamboo
    Mar 14, 2019 at 11:37

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