Q:

What time of year to mulch/feed trees?

This answer recommends putting down a layer of compost mulch in the fall in order to slowly feed the tree. (In addition to the unmentioned weed control and moisture regulation.)
It makes sense to me -- I was ready to go out and spread compost around my apple, peach, and maple trees a couple of weeks ago until I did some reading that suggested I should wait until April.
The advice in books and elsewhere said that we should feed/mulch trees in spring in order to prevent stimulating new growth just before cold weather (to avoid winter kill). In addition, they caution against mulching heading into winter to avoid providing a home for mice or voles.
Which is correct? Is a late fall feeding really going to stimulate any growth?

A:

I'm no scientist or horticultural expert, the below is just my take on how compost works when used as a mulch layer. "My take" comes from experience (for what that's worth), listening to people I respect and admire, reading -- both information that aligns with my beliefs on the subject and with information that I don't necessarily agree with.
Compost slowly feeds a tree (plant, shrub, etc) overtime, it does not provide an instant, "artificial" boost like some chemical fertilizers can eg high N-P-K numbers or totally unbalanced numbers. The benefits of using natural products like compost build up overtime, they are not instant fixes...
Therefore putting down a compost "mulch" layer late in the season (just prior to Winter) isn't going to stimulate new growth. Why?

By the time the compost starts to work its way down into the soil (penetrate down into the root zone), the tree (plant, shrub, etc) has entered dormancy and isn't going to be stimulated into "life" by such a natural product. Natural products by their very nature don't have crazy high N-P-K values, etc...
When the tree (plant, shrub, etc) begins to wake from dormancy, late Winter, early Spring, the elements and micro-nutrients from the compost are there, ready for the tree to start using immediately.
On the other hand, if you put down a compost "mulch" layer in the Spring, it's going to take a few weeks before the tree (plant, shrub, etc) is able to start making use of the elements and micro-nutrients found within.

I'm not saying that's a bad thing, as I believe feeding plants naturally at anytime of the year isn't the worse thing one can do. Yes, there are certain times of the year (Springtime "generally" being a good time) that are better for feeding than others, but when feeding naturally the dangers from "wrong" time of year feeding are a lot less...

Someone can correct me if I'm wrong on this, but mice, voles, etc don't set-up homes in "compost" mulch layers, whereas wood-chip mulches and shredded fall leaves used as a mulch layer can be a different story. Why?

It is my understanding that mulches made from wood-chips and shredded fall leaves greatly resemble the coverings used by mice, voles, etc in nature, hence those types of mulches being more attractive to those creatures. Whereas compost "mulch" is just like an additional thin layer of "soil" on the surface.

A:

Mike is correct. In general, a slow release fertilizer applied in the fall in moderate amounts will not produce a late flush of growth. This is particularly true of compost since the rate that it releases nutrients into the soil is controlled by temperature. As it get colder, it releases less.
Let me note two things about fertilizing trees most people are not aware of:

Most established trees have a root systems that extend two to three times their spread, and their most vigorous, effective feeder roots lie outside the drip line.
Most established trees have very little need for phosphorus or potassium. Mostly, they need nitrogen since it easily leaches out of the soil when it rains. (Tree and shrub fertilizers mixes are usually tuned for young and newly planted trees, which benefit from more phosphorus and potassium.)

Consequently, a good lawn fertilizing program is also a good fertilizing program for the established trees in the landscape.

