Question
I have an oak tree that is dropping significant amounts of black granuals - very simialr to coffee grounds.  It started about 3-4 weeks ago and continues still.  It leaves brown stains the concrete below (much like coffee) and causes our hot tub to turn very brown as well.  

What is this, how long will it last, is there anything we can do to stop it?  Thanks

Answer
Could be a couple of things--caterpillar feeding on the leaves and secrete droppings (frass) waste material. You should see signs of the leaves that have been eaten on the tree.

It could also be the immature acorns aborting.

Acorns are the fruit of the oak tree and take two years to mature. IF the weather conditions are not normal the acorn will abort early. Oaks contain a chemical (The tannic acid or tannin ) that will stain where they lay. These nuts are high in tannins. Think of acorns like red wine which is
also high in tannins and can leave dark stains on clothing or
carpeting.

When the tannins are spread over a porous surface, like concrete, the
result is a dark stain with a greenish or golden cast that looks like
someone threw a pot of tea on your driveway. That is what is happening when the acorns fall onto your driveway and then the rain washes over them.

There are a couple of remedies you might try.

One the safest is to use a product called CLR.

This mixed with a little warm water and brushed on the area should
remove some of the stain. It might require a couple applications, or
you could try it full strength.

As with anything you should test a small inconspicuous area of your
driveway first to make sure there is no adverse effect.

We feel the best way to remove the stain is to use Muriatic Acid.

Usually available at hardware, paint or masonry supply stores.

This is a product that is extremely corrosive and poisonous, care
should be taken when using Muriatic Acid. Wear vinyl gloves, eye
protection and old clothes and shoes.

Mix 1 part acid to 20 parts water. It is important to add the acid to
the water. You can gradually add acid to the mixture to strengthen it.

Apply the acid with a scrub brush. The solution should bubble when
applied to the concrete, if its mixed strong enough.

Let it set one to two minutes and then flush with plenty of water from the garden hose.

They will stop dropping when the acorns set for the year and mature as acorns.

If it is caterpillars eating the foliage they normally have one cycle per year so the feeding will stop soon and the tree will put on new leaves. You can treat the tree with an insecticide but you would have needed to treat as soon as the caterpillars appeared to control them. IF it is caterpillars I would just wait it out since they should be about finished with their feeding stage.