[Green]

We have the rule of thumb for pre-emergent applications: make sure it's applied and watered in before the average soil temp in the top inch or two hits 55F for 4 days in a row.

Ok, but what about cold temps after germination? Is newly germinated crabgrass susceptible? Will it get killed? Or will it be protected by the turfgrass? Or is it somehow resistant to cold damage in the seedling stage? How cold does it have to get and for how long to have an impact?

Hardly anyone talks about this. @thegrassfactor once mentioned a night of mild frost hitting 38F might kill crabgrass, or he was hoping it would (back when he did lawn applications). I found no other info online about this, including articles from university and state extensions, etc.

Clearly I'm not the only one wondering about this:

If this really happens at temps in the 30s, it would be a useful bit of info to know, and might buy people extra time.

Has anyone seen this happen, read a study, or know a bit about it?

 

[PlanetBeen]

I was actually wondering this myself. I feel like I was late with my pre emergent and it had me wondering what happens when we get a week or 2 of unseasonable weather (like we did, 60s, 70s and even up to 80 something one day!) then go back to seasonable weather and even back to nights in the 30s with a couple of freeze warnings.

Any chance it started to germinate but then got knocked out by 'second winter' ? Well, I can dream at least...

 

[macattack]

Sounds like an experiment is needed. I haven't seen any come up yet in the mulched beds, which may get overspray of pre-emergent. Who has time to grow a separate pot of crabgrass?

 

[tgreen]

Great question Green. I have been wondering this same thing. You could imagine scenarios where the air temp gets below 32 but the crab doesn't die. For example, maybe the ground temp is well above freezing and there is one night where air temps gets to 32. In that situation, maybe it doesn't die b/c the grass cover insulates the plant.

However, if you get a hard freeze below 28, particularly for more than one night then I think it's a really safe bet that any crab seedlings wouldn't survive. I don't have a lot of experience with crab (thankfully) but around here I notice that mature crab appears to be dead after a hard freeze in the fall so I assume a seedling would definitely be toast.