Editing just to say thanks to everyone who has contributed. The responses make perfect sense!
In: Biology
The simplest explanation, grass can’t grow through even a relatively thin mat of leaves. Grass is never trying to grow when covered in snow, but it IS trying to grow in spring, and leaves can still be around in spring.
If you anything that covers up plants, they will try to grow and find sunlight, but eventually will die. If you rake up those matted and wet leaf piles in mid- late- spring, you will find green shoots of grass trying to grow underneath. However, they only have so much energy and are unable to push directly up through the leaves, so once that stored energy is gone, and they have not yet found light, the grass dies.
This is literally the exact process that most weed barriers use. Weed barriers ONLY cut out light, they let water through and do not change soil conditions, but you will find that after awhile, there are just no more weeks growing because they all tried, and died. Grass is still just a plant.
Turf generally has a four things it does in a year.
1) Actively growing. Times are good, weather is warm, the turf is trying to grow and spread as much as possible.
2) Actively storing energy, The seasons are changing and the turf is preparing for winter by loading up on carbohydrate reserves in the perennial parts of the grass (stems, stolons, rhizomes.)
3) Dormancy. The grass is too cold to grow and done storing energy, it goes to sleep until the weather turns again. Turf turns brown.
4) Preparing to grow. The weather is warming up, but not warm enough to grow. The turf will turn from brown to green using all the stored energy it saved up in the fall. It does a combo of use the stored energy to grow and starts storing more for when it is time to grow.
All that to say, that in dormancy, the turf can handle being covered for months, it is hibernating and doesn’t need any sunlight.
Leaves (any kind of shade really) can be really harmful because the turf needs the sunlight in all the other times of year. Whether it is trying to grow, or storing energy, shade is a major hinderance. Grass will spend all of its energy trying to get above the shade thus exhausting its energy reserves. That is why turf in the shade grows much longer and less dense that turf in full sun. Any trampoline owners out there? It is trying to get more leaf surface area to do more photosynthesis.
Oftentimes, a late snowfall can be really devastating for turf as well. The turf has used up its energy reserves to come out of dormancy and then gets covered again for an extended period of time can wipe it out.
Of course, not all turf is the same and there are major differences in species and varieties, but for the most part, this is true of all turf grasses.
I think the last thing to add to the aforementioned comments is that, in a leaf pile there is a tonn of life, insects and worms eating away at things, breaking down the leaves, which releases chemicals from the leaves as well as releasing their own faeces and chemicals seeping onto the grass.
Snow is just water, and most northerly grass is well conditioned to handle dormant months under snow.
Tree leaves aren’t just water, and may even be intentionally poisonous. Plants aren’t always passive observers to the forest, they’re active participants and they compete with eachother.
Sometimes this competition is quite vicious, and many trees have evolved leaves that don’t just rot away to nothing on the forest floor, but rather salt the earth below them to exterminate any nearby competition.
The chemicals (and if it’s big enough, the heat) released by a rotting leaf pile can kill any living plants nearby.
A few things,
In cold temps, plants go into hibernation mode. It uses the energy it stored up to ride out the covering of snow.
Also leaves are highly acidic. To many in a given area will change the properties of the soil.
Finally, is the leaf still living?
Fresh leaves still absorb sunlight, even off the tree!
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