Question
is horse manure good as fertilizer for flowers?

Answer
Yes with some modification and reservation.

Horse manure may be a good fertilizer, but it is not a high source of phosphorous which flowering plants generally consume in large amounts.

A good flower fertilizer will have ratios 1-2-2 or 1-2-1 (such as 5-10-10, 5-10-5, or 15-30-15, for example).

Horse manure has a ratio close to 2-1-2 (usually rather low around 0.7-0.3-0.6). If you manually add another type of (organic) fertilizer which is high phosphorous, then you can use the combination with success.

Make sure the horse manure is well aged so that the soluable salts are washed away. It should have been allowed to lie on the farm for several years where rain could get to it. If applied fresh, it may kill flowers due to high salt contents.

Try to combine the horse manure with some bone meal which is high in phosphorous. Bone meal is easily available in 5 lbs bags at larger garden centers (in the organic fertilizer isle).

-- Kenneth