For The Union-Tribune
It’s that time: Buy fertilizer. Apply it per package directions. Water deeply.
That is that. Simple and easy.
But “simple and easy” isn’t always the case when it comes to fertilizing decisions and implementation. Like us, our roses need nourishment to grow and thrive. Should we be on a Mediterranean, low carb, keto, paleo or plant-based diet? Should we eat at equally spaced intervals, fast or do intermittent fasting? Should we eat only organic foods? Experts differ on what constitutes a healthy diet for us.
Similarly, we find numerous conflicting views, opinions, directives and methods when it comes to our plants’ “diet” and fertilization requirements. In San Diego, many serious rosarians fertilize with a plethora of fertilizers every two weeks commencing in mid-February through to mid-October. Other rose growers find they can grow beautiful roses by fertilizing with a balanced organic granular product applied just two or three times a year. Some rosarians use only organic fertilizers. Others alternate organic fertilizers with fast-release synthetic fertilizers throughout the growing season. Some growers swear by slow-release granules. One of my good friends, an avid exhibitor, has his irrigation system set up to deliver a dilute amount of liquid-soluble fertliizer to his roses each time they are watered.
What is the best “diet plan” for our roses?
Soil test
Over feeding and under feeding can be problematic not only for us but also for our plants. I wanted to make my fertilizing decisions simpler and easier, so several years ago I sent a soil sample to a soil lab to get my soil’s pH and a scientific recommendation of how much and what fertilizers to add to the soil. The results indicated that my soil pH was a high 7.5, and there were excessive amounts of phosphorous, potassium, sulfur, zinc, iron and copper in my soil, even though I used only organic products that did not contain high amounts of these nutrients. It was a matter of accumulating too much year after year.
The lab’s suggested fertilizer application for my soil was solely nitrogen. I could use ammonium sulfate 21-0-0, urea 46-0-0 or blood meal 12-0-0. I picked the organic option and fertilized solely with blood meal. To lower the pH, the lab suggested applying sulfur in the fall or in spring.
Fertilizers are not plant food. Plants ‘feed’ themselves
When we add fertilizers and amendments to our soil, we are not, strictly speaking, “feeding” our roses. Organic fertilizers and amendments that are incorporated and watered into our soil need to be broken down by the activity of millions of many species of tiny organisms, microbes, fungi, insects and earthworms in the soil. These nutrients are then made available and utilized over several months by our plants. Synthetic and water-dissolved fertilizers are quickly available to our plants. But it is our plants that “feed” themselves by uptaking the available nutrients in the soil and producing their own food in the form of sugars through the process of photosynthesis.
The primary or macronutrients which are essential for plant growth are nitrogen, phosphorus and potassium (NPK). Fertilizers must disclose the amount of nitrogen, phosphorus and potassium on the products’ labels. The first number is always nitrogen (N), the second number is always phosphorus (P) and the third number is always potassium (K). The secondary nutrients and micronutrients if included are listed on the label.
Organic fertilizers often include blood meal, bone meal and fish meal. Keep an eye on your dogs, as these smells are strong for a few days after application and may prove to be irresistible to them. (Rita Perwich)Nitrogen is necessary for lush, green growth. It is the one primary nutrient that must be added to the soil periodically throughout the growing season. because nitrogen is very mobile and depletes quickly. But an overuse of nitrogen must be avoided as it can burn tender roots and promote excessive vegetative growth which can attract sucking insects like aphids.
Phosphorus promotes big blooms and is essential for flowering and strong root growth. For this reason, rose gardeners tend to apply fertilizers with high amounts of phosphorus but we can easily oversupply our soil with phosphorus since phosphorus can stay put in the soil for years. We also jeopardize the mycorrhizal associations in our soil with the excessive use of phosphate.
Potassium helps produce sturdy plants and increases plants’ resistance to stress but overapplication of potassium (and too much of any nutrient) can cause other necessary nutrients to become unavailable to the plant.
Plant nutrients that are needed in much lower quantities are boron, calcium, chlorine, copper, iron, magnesium, manganese, molybdenum, sulfur and zinc. Magnesium is often over-applied in the rose garden because many rosarians mistakenly believe that the addition of epsom salts (magnesium sulfate) promotes new basal breaks.
Why soil pH matters
An incorrect pH prevents our plants from properly taking nutrients from even a nutrient rich soil. The ideal pH for roses is a soil that tests between 6.0 and 6.5. The pH can be decreased with sulfur and increased with the use of lime.
Fertilizers: organic, synthetic, granules, liquids
We are at the nursery. What to buy? If we are trying to provide general nutrition, a granular product derived from multiple organic ingredients such as bone meal, blood meal, kelp meal, fish meal, bat guano, cottonseed meal and alfalfa meal is a good choice. The NPK on these products is usually in the lower zones such as 3-4-3 or 5-6-3.
Fertilizers can also be bought as a single ingredient. Cottonseed meal (6-2-1) is a good choice if you have an alkaline soil as this fertilizer helps acidify the soil and aids in production of friendly soil bacteria. Alfalfa meal (2.5-0.5-2.5) contains triacontanol, a natural growth stimulant that increases plant vigor, bloom production and basal breaks. It is a rich source of trace elements and an excellent soil conditioner.
Application
Fertilizer sitting on top of the soil provides no nutrients to our plants. Granular products must be scratched lightly into the soil and watered deeply down into the root zone. Follow label directions for how much and how often to apply, always erring on the side of less rather than more.
Granular applications of fertilizer can be supplemented periodically with water-soluble organics. Fish hydrolysate (2-3-0) provides nutrients, vitamins, hormones and micronutrients and maintains and increases soil fungal populations. Liquid seaweed (1.5-0.5-2.5) adds trace elements, growth hormones and vitamins that improve overall plant growth and reduce plant stress from drought.
Take caution when using fertilizers with high NPK numbers like 20-20-20 or 16-16-16. These synthetic fertilizers, are usually cheaper than organics, provide rapid results, but can more easily burn our plants. Do not fertilize on very hot days and be sure to water your plants before and after fertilizing. Unlike organic fertilizers that can improve soil structure and soil condition, chemical salts can build up in the soil with constant use so alternate with organic products for your soil’s health.
Timing of application is important
A plant with no foliage is not drawing up water or nutrients from the soil. It is only when the plant has leaves, photosynthesis is taking place, and the leaves are transpiring that the plant draws up water. The nutrients that are in the soil are drawn up with the water.
In San Diego, we apply organic granules when the soil warms up in mid- to late-February so nutrients will be available to the plants when they have several inches of foliage in March. We stop fertilizing in October when plants’ active growing cycle is slowing down. Chemical and liquid-soluble fertilizers should be applied only when the plant has several inches of new growth. Liquid fertilizers can by our plants’ roots before the plant can utilize them and rapidly leach into the soil when we get heavy rains.
My follow-up soil test
A recommended regimen works only when it is implemented so a follow up soil test felt like stepping on a scale. How did I do? The sulfur lowered my soil’s pH to a much improved 6.8, and my soil’s potassium level is lower. But the phosphorous, potassium, sulfur, zinc, iron and copper are still in the high range.
Don’t judge me, but I broke my roses’ ‘diet’
Why are my soil’s phosphorus and some micronutrients still too high? It could be because phosphorus remains in the soil for a long time. It could also be because we love to feed the ones we love. Here’s my confession. Last year I broke my roses’ bland single-ingredient blood meal diet, and I “fed” them a few “rose superfoods.” I substituted the 12-0-0 blood meal for 6-2-1 cottonseed meal and 2.5-0.5-2.5 alfalfa meal. Then, between granular ‘feedings’, I applied 2-3-0 liquid fish hydrolysate and 1.5-0.5-2.5 seaweed to my soil. I imagined my roses loved the healthy ‘diet’ additions as much as I love healthy berries and dark chocolate.
Fertilizing can be simple and easy. But for that to happen we need to have a plan and follow the science. When we dial in our human projections on what and when we think our roses want to “eat,” fertilizing can get complicated, dizzyingly crazy and not so simple and easy.
Perwich is a member of the San Diego Rose Society, a Consulting Rosarian and a Master Gardener with UC Cooperative Extension. For more information, visit rose-harmony.com.
