[Lewis Thomson (OP)]

Paul has presented this gardening dilemma to us,

"Weeds don't seem to be deciduous. I was wondering if I could convince my wife that I am saving the planet by not weeding the garden. Do weeds sequester carbon? Besides, weed cover also retains moisture on the ground. If I weed the garden, the wet soil drys out."

Do you think weeds have benefits to gardening? Leave your thoughts in the comments below.

[Bored chemist]

A rose bush in a cabbage field is a weed.
A cabbage in a rose garden is a weed.
There is no definition of a weed based on how it interacts with the environment, it's just based on human whim.
So, if you decide that you will let the nettles grow, they are no longer a weed (unless your wife disagrees)

Weeds don't seem to be deciduous.

There are plenty of annual weeds.
This is probably the best studied.
https://en.wikipedia.org/wiki/Arabidopsis_thaliana

[evan_au]

There is a long-standing practice of letting a field "lie fallow", allowing weeds to grow and restore the soil.
- Clover captures nitrogen, so the field requires less artificial fertilizer
- A mixed ground cover is better for the soil microbes, and bees get a supply of food for longer than with a mono-crop

Having a ground cover reduces soil erosion from rain or wind.
- Apparently, much of our fertile soil resulted from rocks ground up by glaciers in the last ice age
- The next ice age is a long time to wait to restore our soil...

[alancalverd]

A weed is a plant you didn't pay for. A local charity has just bought an arable field and is returning it to its natural state by doing nothing for a few years, in the expectation that it will attract insects, birds, reptiles and a few mammals that are in danger of extinction.The only problem seems to be pheasants that run away from a nearby shoot (who wouldn't?) and eat the young plants, but the field already looks a lot more interesting than when it was covered with wheat.

[Bored chemist]

A weed is a plant you didn't pay for.

So the poppies I got from my mum are weeds?
A weed is a plant  where it is not wanted.

[walnutclose]

"Weeds don't seem to be deciduous. I was wondering if I could convince my wife that I am saving the planet by not weeding the garden. Do weeds sequester carbon? Besides, weed cover also retains moisture on the ground. If I weed the garden, the wet soil drys out."

No, you can't tell your wife that you are contributing meaningfully to carbon sequestration by not weeding the garden.   If you were to stop tilling the garden, and grow high biomass weeds and just let them decompose into the soil naturally, you could potentially sequester a little carbon, but the numbers are not really with you.  If your garden is 1000 ft² that's 1/100 of a hectare.   Good cover cropping might sequester 3 tons CO₂ equivalent in organic carbon per hectare, so you'd get .03 tons, or 60 lbs CO₂ equivalent.   To put that in perspective, it's the carbon equivalent of not burning 3 1/2 gallons of gasoline.   If you're an average American driver, that's about 1% of your gasoline usage for the year.

And that's if you let high biomass weeds completely overwhelm your vegetables and flowers, and don't till the soil at all - grow a tall grass prairie, in other words.   If you till it, the sequestration is WAY less, because you chop up the organic matter and expose it to lots of oxygen, so it mostly just gets decomposed into CO₂ and released back into the atmosphere.