I tried to do some searching but couldn't find much but I believe that UCGs have little caffeine left in them. Usually all the caffeine is in the liquid. If I find something to support that or the opposite I'll post back.

As far as what happen this spring you may of just had to much of a good thing. The UCGs might of just been trying to decompose and took up any free nutrients in the ground. I'm sure the bed will be a lot nicer this next spring.

Caffeine, like the acidity in coffee is water soluble, so the caffeine and acidity are mostly in the coffee, with little remaining in the grounds.

Most of what I can find has to do with the affects of caffeine on slugs and snails but the few studies available about caffeine in soil, that do not charge, indicate that you would need a fairly large amount of caffeine in your soil to cause adverse affects. I've not calculated the milligrams per liter into a weight per pound of soil but it is significant. This is probably not something that most of us adding something less that a couple of hundred pounds per week need to be concerned about.

My searching of the web for research on the subject found the following data points.

Coffee grounds have about .13% caffeine. Ground arabica has about 1.5%, so about 90% of the caffeine gets washed out of coffee grounds.

Coffee plants in coffee plantations exhibit diminished productivity over time due to buildup of caffeine in the soil.

At levels of less than 1000 uM of caffeine, mung beans had a reduced rate of root growth. Between 1000 and 2000 uM root growth was very inhibited. Above 2000 uM there was no root growth. Caffeine inhibits root growth.

There is a bacterium that digests caffeine.

This all leads me to believe that UCG should not be used in gardens in springtime for many types of plants. From my observations, corn seemed to grow well in raw UCG, but the beans did not do well. Carrots seemed to have difficulty with germination, but afterward grow ok.

Now is probably the best time to amend the soil with UCG. Otherwise, they go in the compost pile.

Note that I pick up about 1 cubic foot of UCG a day, so I'm talking about large amounts of UCG in the soil.

I agree with smokensqueal. You buried undecomposed organic material and the process of decomposition underground requires huge amounts of nitrogen. The decomposition microbes take all the nitrogen they can get from the soil. Coffee grounds have a bunch but after that they rob it from the surrounding soil.

Next time use coffee grounds on the surface, like a fertilizer (which is what it is). Use one heaping handful per month scattered under the canopy of each shrub. Then the microbes can get their nitrogen from the air.

@idaho_gardener "Too much caffeine will inhibit root growth."

Perhaps that is why the Sansevieria in my office did so well. I had a Sansevieria in my office which got nothing but stale black coffee for thirteen years and looked just fine for an office plant. Ordinarily we would expect an otherwise uncared for thirteen year old Sansevieria to become root bound and start pushing itself from the pot, dropping dead outside leaves and present other problems. When it got nothing but coffee it did none of these things.

After my retirement I took it home, repotted it and treated it with water and it began to show signs of vigorous life. Being a rare variety I sold it and a fine pot for $32.