Still learning new things after 50+ years of gardening.

Just read James Wong's article on
https://www.theguardian.com/lifeandstyle/2016/oct/23/coffee-grounds-are-not-good-for-plants-its-a-myth.
So this seems to have come from an auto toxicity of coffee plants and a resultant degradation of the soil for coffee growing in places like Latin America. This has occurred due to tens of years of monoculture. It seems that some of the aromatic herbs like sage will help to ameliorate this problem.
I think that this is a classic example of why monoculture is a bad way of cultivating plants.  Not only do we get depletion of specific nutrients and an increase in pests and diseases but also a build up of potentially growth retarding chemicals. 

It is also an example of the danger of extrapolating scientific results without carefully considering whether this is appropriate. 

In Coffea arabica growing regions of Brazil, coffee fruit peel is composted and used on vegetables, fruit trees and for coffee culture itself not only as an organic fertiliser but also as a weed suppressant.  It seems to slow the growth of fine roots.

There are relatively high concentrations of phenols, flavonoids and caffeine found in extracts.  It is amazing what we find good to drink! One or more of these compounds may have allelopathic properties.  It has been found that coffee bean water extracts and dry powder decrease germination in certain plants.  So needs to be pretty concentrated then?  In other plants there is less germination but stronger seedlings than controls.  This has been interpreted as coffee water extracts and lower dose dry powder have allelopathic effects on some seedlings.  Higher doses of incorporated coffee powder has a depressing allelopathic effect on some  plant's growth. 

I would not really worry about putting coffee grounds on growing beds for at least two reasons. You have just put them through a procedure that removes lots of the chemicals from the bean when it has been percolated. The research papers that I have read use quite concentrated coffee extracts to get an effect.    Secondly, you have not been putting coffee on growing areas for tens of years and finally you are not growing a monoculture of coffee. 

I put my coffee grounds on the compost heap. They are relatively rich in nitrogen even after percolation and this means they will help to accelerate decomposition.

I have used comfrey in the same way as James Wong did coffee grounds in the newspaper article and got similar results. I think that his results could have something to do with too much nitrogen rather than the allelopathic effects of coffee grounds. I would not repeatedly put any chemical on growing areas like this because plants only need tiny amounts of nutrients from the soil.

Are celery plants perennial?  There are lots of vegetable plants that are grown as annuals that are biennials or perennials.  It seems that celery is one of them.  It dies after it has flowered so if you can stop it from flowering, you can keep if for several years.  I find it difficult to stop it flowering in the first year and am delighted when I can pick some for Christmas dinner as I did this year.