We often refer to the application of plant fertilizer as plant 'food' but that is not accurate.  A plant derives its nutrients through the absorption of minerals and elements in the soil in combination with the miracle we generally refer to as photosynthesis.  

Whatever.

Truth is that this is the last week of the growing season that you should fertilize frost hardy plants like roses, shrubs, trees and the like.  If you fertilize later in the year you can encourage new growth that will be subject to winter damage as the newest growth will not have had a chance to harden off before winter.  

There is a 3 number analysis on every package of fertilizer sold in Canada.  They always represent the % of nitrogen, phosphorous and potassium in the package.  Listed elsewhere on the package will be additional elements or minerals that are in the product.  

Whether you use a synthetic or natural fertilizer is of no consequence to your plants, as long as they receive the nutrients most needed for great garden performance.  The instructions on the package are important and I urge you to read them.  Look for special fertilizers for plants with peculiar needs, like 'clematis and vine' or 'rhododendron and azalea' or tomato and fruiting vegetable' fertilizers.  All of these have been formulated with the specific needs of each plant in mind.