The use of rose fertilizers ultimately comes down to what you want from your roses. The fundamental difference between synthetic fertilizers and those which are organic is that synthetics feed the plant while organics feed the soil which in turn feeds the plant. Knowing this and knowing what you want from your roses determines which way you want to proceed.
Synthetic fertilizers are high in soluble nitrates - the nitrogen form used directly by the plant. Using these fertilizers, then, will create an immediate uptake of this nitrogen, thus creating a burst of fast growth, and producing a plethora of plant-sucking rose pests like aphids, thrips, et al. which are attracted to this nitrogen and tender new growth. The advantage to using soluble nitrates as fertilizers is that you can control when you want your roses to bloom, how long you want the stems to grow, and a host of other things that are of benefit to the rosarian who wants to exhibit his or her roses at a specified time.
I realized that lilies and sweet peas were going to be relatively easy to grow. Lilies were exceptionally hardy, just plunk them into the ground and they would succeed. Sweet peas could be planted in early spring (zone 8) and they would sprout in a matter of weeks. And roses, well.digging a hole and putting them in was simple enough. What more would they need?
On the down side, it requires you to apply more pesticides to control the onslaught of pest insects, it creates excessive growth at times when freezing temperatures may cause die-back, it leaves undesirable salts in the soil, it shortens the life of the rose, and it bypasses The Nitrogen Cycle - the way Nature intended to fertilize plants - thus reducing microbial activity in the soil which is eventually detrimental to the garden as a whole.
It is for these drawbacks that I no longer use synthetic fertilizers in my rose fertilization program, although I did for almost 25 years before I learned my lessons. Many of the roses I grow have been found thriving at abandoned grave yards and home sites with no supplemental fertilization of any kind. Thus, I prefer to blend organic materials necessary to sustain rose growth over a period of years. This blend is adapted for my garden, and may not be what is needed in yours or others. (I know this blend is good for my garden because I perform regular soil tests - something I highly recommend before you purchase or blend your own rose fertilizer.)
Here's a copy of my Spring and Fall Rose Tonic. It is a modification of a recipe passed to me by a very wise and 83-years young rosarian. It assumes your soil pH ranges from 6.5 to 7.5, clay-based and/or rich in calcium.
8 parts alfalfa meal
2 parts cottonseed meal (arsenic-free)
2 parts rock phosphate (or colloidal phosphate; but not super phosphate)
2 parts bone meal
1 part blood meal
1 part Epsom salts (magnesium sulfate)
Thoroughly combine ingredients and apply 1 cup (.24 l) of mixture for each foot (30 cm) of shrub height. Lightly scratch the mixture into the soil using caution not to harm the roots. Water-in thoroughly.
This mixture is good in the spring and the fall, and can be applied as a 'tonic' to all blooming perennials, roses, azaleas and fruit-producing trees and plants.
If soil pH is above 7.5 (and it shouldn't be), reduce the Epsom salts by half to reduce the possibility of applying too much magnesium.
For new plantings, add 1 part Greensand (0-1-5) to the Fall Tonic
The NPK of the constituents varies by manufacturer; however, on average these proportions will create a 50-110-20 (or approx. 2-4-1 NPK ratio) slow-release, environment-safe fertilizer. It will not "burn" the foliage or contaminate the soil.
Blood and bone meals may attract dogs and cats. If this is a problem, repel with pepper spray around the watering well.
Feather meal may be substituted for alfalfa meal, but the quantities should be cut in half. Feather meal is also quicker to decompose and will not provide sustained nitrogen release beyond about 21 days. Furthermore, alfalfa has micronutrients not found in feather meal.
Alfalfa meal is quite dusty; a dust mask should be worn when mixing the ingredients.
I apply this tonic immediately after spring and fall pruning (here, late Feb and late Aug). It is so effective, supplemental fertilizations are not required but once-a-month in the spring - until I let the roses rest during summer's heat. For supplemental fertilizations, I use a blend of fish emulsion, kelp liquid concentrate, and un-sulfured molasses @ 2 Tbsp, 1 Tbsp, and 1 Tbsp (30ml, 15ml, 15ml) respectively per gallon of water and pour this around the watering well of each rose.
It is best, if you have a lot of roses, to purchase the ingredients in bulk from the farm/ranch or feed store vis-à-vis the nursery or mailorder. For example, alfalfa meal at the feed store costs about $8 for 40 lbs. At the nursery, it costs $7 for 4 lbs. - and it's the same stuff, just packaged with a prettier label. Use caution when purchasing from a feed store, however, to ensure the alfalfa does not contain salt, as this will be detrimental to your soil. (Try a taste test.)
And if you grow roses in pots or sell them commercially, sprinkle 1 or 2 cups of mixture around the top of the pot. When you water the plants, the fertilizer acts like a timed-release mixture that can cost much, much more.
It should also be noted that some rosarians who live in cold climates report they have reduced aphid and thrips infestations if they withhold fertilizing until after first flush in the spring, thus letting the rose generate growth and blooms from the stored energy in the roots (e.g., like bulb perennials). This would not work for us who live in warmer climates because first flush may not end until May or June, when our temperatures quite regularly get above 95°F(35°C) and our roses like to rest during summer's heat.
