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Grass gernmination and soil temperature

Discussion in 'Homeowner Assistance Forum' started by Alto101, Apr 1, 2004.

  1. Alto101

    Alto101 LawnSite Member
    Posts: 18

    I have read that grass seed germinates when the soil temperature is around 55 degrees. To estimate the soil temperature I know I can take the high temperature and the low temperature and divide by two. My question is what will happen to grass seed at a slightly lower soil temperature, say 52 degrees? Will the grass seed not germinate, will it just take longer to germinate, will some seeds germinate while others lay domrmant, or something else that I have not considered. I want to do some overseeding soon but don't want to worry about keeping the seeds moist and protected from the birds if they will not germinate. The 10 day forecast for my region looks like a lot of days in the mid 60's and lows in the low 40's so average temperatures of 50-55.
  2. SWD

    SWD LawnSite Senior Member
    Posts: 989

    Dependent upon species of grass seed, most northern grasses begin germination at or around 59*F. They typically stop germination at or around 80*F.
    Ambient air temperature usually has no impact on soil temperatures - you actually need to measure in the 1.5" to 2" depth range. Check the site with the most sun at or around 8 am. Soil is a extremely good insulator, and when you factor in the growing/climate conditions of your particular zone, the soil temps may lag several weeks.
    With regards to the actual seed itself and non-germination, unless the soil is kept in a saturated condition and the seed rots, then nothing. There is a way of implementing dormant seeding to an area-I do it all the time.
  3. philsaunders

    philsaunders LawnSite Member
    Posts: 5

    Pardon me if I missed posts on this Title, but last one I saw said little about process. Late winter seeding has a couple apparent benefits: moisture and fewer weeds. Issue is what happens to seed prior to germination conditions. Cool temperature would seem to limit rot risk. So, what else may happen?
  4. Smallaxe

    Smallaxe LawnSite Fanatic
    Posts: 10,081

    Cool season grasses are said to germinate at 50F. and CG 55F.

    Air temps mean little, as long as they are not extreme enough to affect soil temps... Sunlight on the other hand has a huge impact... baking sun on thin grass will get CG to popup almost instantly, once the growing season gets underway...

    The 10 day forecast in the Spring is meaningless, in the sense that it is too cold for seed to germinate in 10 days... freeze/thaw cycles 'set' the seed, all the while soaking up moisture and otherwise preparing for germination... When that magic moment of correct temps are met; the seed is ready to go...

    waiting until the temps are correct before even putting the seed on the ground,,, means the seed only now starts preparing for germination,,, annd may mean its going to be late...

    never too early to seed... :)
  5. philsaunders

    philsaunders LawnSite Member
    Posts: 5


    Thank you for such a quick reply. As to my question about the general process, I share here a URL for a website that is quite academic: http://www.lawn-care-academy.com/grass-seed-germination.html. Your reply is more practical.
    As to timing and temperature, as you address, your answer seems complete. However, it brings up the next question: how soon is a combination pre-emergent broadleaf herbicide/fertilizer advisable? Clearly, when all the seed germinates that is going to, is likely the answer; but to be practical, do you wait until the shoots sprout about one-half inch, or longer?
  6. Smallaxe

    Smallaxe LawnSite Fanatic
    Posts: 10,081

    I'm sure that you know not to kill your seedlings with Pre-M, but you may be tempted to give up on your germination, becuz of fear... fear that your seeds won't germinate and the CG will... we have such fear of CG here in CentroWisco that pre-m is put down before Memorial Day,, but I've never seen a CG start in my full sun loose soil garden, before Memorial Day,,, so how can properly managed turf be hot enough to germinate a CG plant???
    You have to make a Judgement Call... :)

    My suggestion is to give priority to growing grass rather than killing seeds...
    I never use pre-m, on turf,, becuz if there are no barespots in the lawn and the grass is cut high during the HOT season, CG is NEVER an issue for me...

    Young seedlings spend a huge part of their development growing root after they put up their leaf... pre-m being a root inhibitor will still damage the long term well-being of the grass...
  7. Smallaxe

    Smallaxe LawnSite Fanatic
    Posts: 10,081

    Here's a valueable paragraph from that link you posted...
    generally both sod and seed are way over-watered, thereby not only creating abaerobic 'rot' in the soil, but also damaging any possility of building soil structure that is absolutely necessary for proper growth of the seedling AFTER gernination... worth analyzing for a moment... enjoy... :)

    * "The seed operates in an anaerobic manner during the initial stages of water absorption at the start of germination. This continues until the seed coat ruptures. Once the seed coat ruptures the seed moves from being anaerobic (not requiring oxygen) to aerobic (requires a steady supply of oxygen). This process of oxygen exchange in plants is called “transpiration.” ..." *
  8. philsaunders

    philsaunders LawnSite Member
    Posts: 5

    Smallaxe, your posts are indeed thought-provoking. When you point out the risks of over-watering, damaging any possility of building soil structure, you impel me to share that I am over-spreading the seed with top soil and compost.
    So far, I await rain for watering; but what do you think about watering lightly on dry days to aid in raising the soil temperature?
  9. philsaunders

    philsaunders LawnSite Member
    Posts: 5

    Smallaxe, I failed to see your next to last post before responding. Thank you for your advice about "Pre-M." Your information that "root inhibitor will still damage the long term well-being of the grass", makes me feel strongly that I'll wait until after the first mowing, likely in June, before doing the "Pre-M."
  10. Smallaxe

    Smallaxe LawnSite Fanatic
    Posts: 10,081

    I generally soak the seed into the topdressing of soil and compost whenever possible to get it with excellent contact... then let it dry to some degree before I water again...

    If I'm unable to do that, I just let the Spring rains do it for me... Springtime droughts can indeed be a killer, so if you can irrigate, do so,,, just allow drying time inbetween... dry/soak cycles build soil structure too... :)

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