Have you had a soil test done? Did it indicate it needs fertilizer? Are you seeing signs of unhappiness in the plants? If no, then you don't need to fertilize and are as likely to cause issues as to help.

Regardless, don't ever fertilize with quick release fertilizer in fall since it will encourage tender growth that won't have a chance to harden off and will be winter killed.

they are not children ... they dont need to be fed .... ken

Box bursting into new growth at wrong time of year due to fertilization unlikely. Box needing fertilization for best results likely, in any locations where poor soils are often present. Mineral deficient soil conditions can be present merely because a site has been developed.

Contrary to the much publicized recommendation to NOT fertilize landscape plants at time
of planting or for the first year, my research has consistently shown that fertilizing at
planting is VERY beneficial to root growth and establishment. The key is using the correct
fertilizer and at the correct rate and allowing for the much greater VOLUME around the plant in
the landscape. The concept of adding fertilizer proportionate to the volume of the container
growth medium is routinely practiced. However, adding fertilizer proportionate to the volume of
soil in the landscape-planting site is almost never considered. A tree or shrub grows in a volume
of soil in the landscape, just as in a container. I strongly suspect the reason that most studies on
fertilizing landscape trees and shrubs show little or no growth response is because poor root
systems on the test plants and of inadequate consideration of the volume of soil involved. Plants
grown with superior root systems that provide rapid root extension into the surrounding soil
following transplanting show a far greater response to fertilizers. Why? More root surface area
for absorption. Excess fertilizer, especially nitrogen and phosphorus is as detrimental as a
deficiency. The only way to know what the soil on a specific site needs to assist plant growth is
to soil test. Anything else is simply a shot in the dark. If phosphorus is already high, adding
more is detrimental to root growth and establishment.

ESTABLISHMENT OF CONTAINER GROWN PLANTS

https://rootmaker.com/sites/default/files/pdf/establishment.pdf

Fertilizing...or not fertilizing...can get quite complex and extraordinarily specific.

Simple, short answer...slow release is the way to go. Osmocote or Plant-tone are brands to looks for. Particular in late season, avoid high N.

I can think of a very practical reason to consider ensuring adequate fertility as fall approaches...adequate levels of K in plant tissues can increase winter survival.

From the ESTABLISHMENT page linked to above, written by somebody who has done the research:

ALWAYS
soil test and add deficient nutrients prior to planting. AND, always have roughly 25 to 50 pounds
of nitrogen per acre present in the soil, even when planting in September, October or November