This is an exciting time of year for us rose growers. Our roses have leafed out, and our earliest varieties are budding. (The Harison’s Yellow rose — circa 1750s — in the Olivia White Hospice Garden is the first to bud there.) When the nights in May are not freezing (27 degrees or below) any longer and the days are warm, it is time to fertilize your roses.
Finish your spring pruning first. Remember that it takes a rose about two weeks to start making use of the fertilizer and show new growth. So time your fertilizer application about two weeks before your last frost day, June 1-15, depending on where you live in Flagstaff. Since our growing season is short, do not delay. Fertilize as soon as the extended weather forecast predicts lows above freezing, 27 degrees. Fertilize a second time around the first week of August, but no later than six weeks before the first frost.
We are all busy, but do not skip fertilizing even established plants. Our topsoil here on the arid Colorado Plateau is very thin — only a few inches deep. Our topsoil also has very little organic matter with which to enrich your roses and hold water. The rose combines fertilizer compounds with its products of photosynthesis to fuel its growth, repairing winter damage and producing flowers then seeds within the resulting rose hips.
Use a fertilizer that fits your needs. There are advantages to both organic fertilizers and to inorganic fertilizers.
Inorganic fertilizers are made of chemical compounds from non-living sources. Nitrogen (N), Phosphorus (P) and Potassium (K) are the main elements needed for plant growth. The amounts of these elements are expressed as N-P-K numbers, which are the percentage by weight of each of these elements. An example is 11-15-11. These are usually fine granules, and are easier to spread than organic fertilizers. Some of the Nitrogen may be polymer coated for slow release. Their larger N-P-K numbers mean that they will produce more growth in a shorter time. They do not condition the soil, and the binder may remain as a powdery residue.
Organic fertilizers are meals made from living plants, such as kelp and alfalfa meals, and animals, such as chicken manure and worm castings. These meals and manures totally decompose, but slowly and may not provide their nutrients soon enough. Most organic fertilizers usually have low N-P-K numbers, around 5-5-5 or less.
Mycorrhizae are added to some organic fertilizers. These are microscopic fungi which have a symbiotic relationship with the roots of plants, better enabling the roots to exchange water and nutrients. Adding additional mycorrhizae has been studied for over 30 years, and helps replace mycorrhizae lost by garden soil preparation.
Always use a fertilizer with a high Phosphorus (P) level since Phosphorus promotes strong roots and vibrant flowers. Avoid fertilizers with a much higher Nitrogen (N) level as Nitrogen promotes leafy growth at the expense of flower production.
Apply the fertilizer according to package directions in a broad band half the way to the edge of the bush all around. More is not better because it may chemically burn the rose bush, damaging the plant. Find your irrigation emitter or ring, placing it on top of your mulch so that it will not be clogged by the fertilizer. Water the fertilizer in deeply to the edge of the bush. Continue your regular watering schedule to dissolve the fertilizer as soon as possible.
The plant nurseries in town carry a variety of fertilizers. Compare these to find the ones you feel suit your roses best and enjoy the beautiful results.