Just how fertile is your rose-growing knowledge?

There is a lot of information out there on how to grow roses. Unfortunately, some of the information is contradictory. How do we separate good rose culture from rose lore and rose myth? Should we apply epsom salts, mycorrhizae, or phosphorus to our soils … or not? Does rose replant disease exist … or not?

It is difficult to know which of the contradictory truths to believe. They cannot occupy equal space. The best rose practices are based on knowledge about our soil and the science of growing plants.

Test your knowledge. Answer True or False to the following “Rose Believe it or Not” statements:

1) Roses must be sprayed with pesticides and fungicides.

The syrphid fly is a beneficial insect. Its larvae rid roses of insect pests, slugs and caterpillars.

The syrphid fly is a beneficial insect. Its larvae rid roses of insect pests, slugs and caterpillars.

(Rita Perwich)

False. Well-chosen varieties of disease-resistant roses planted in good soil in a sunny location, with good air circulation and adequate water, can thrive and bloom without sprays. As with all plants, the gardener must avoid an infestation by frequently inspecting for pests and manually removing them.

2) The pruning cut must be slanted at a 45-degree angle.

False. Pruning cuts on fruit trees are slanted because of the sap that can ooze over the bud eye. Roses don’t ooze when cut, so it is not necessary to make your cuts at a 45-degree angle. It is, however, important not to leave more than a quarter inch above the cut, otherwise the cane will die back to the bud-eye and will sometimes continue to die back even further down the cane.

3) Organic and chemical fertilizers are equally good.

False. Although organic and chemical fertilizers both provide nutrients for your roses, organic ones have definite advantages. They improve the soil. Chemical fertilizers provide a quick food fix with no benefit to the soil and can burn and damage the plant. If you use chemical fertilizers, alternate with organic fertilizers and add an annual top dressing of organic mulch to the soil.

4) Use high NPK fertilizers, as roses are heavy feeders.

False. Be very cautious when using high NPK (nitrogen-phosphorus-potassium) fertilizers; dilute them as recommended on the label, and water your roses before and after fertilizing. If you have neglected to feed your roses for a while, don’t “catch up” your feeding program by under-diluting your fertilizer, as you will harm your roses and burn them.

"Playboy" is a floribunda with ruffled copper-orange petals. It has yellow stamens that attract bees and other pollinators.

“Playboy” is an eye-catching floribunda with glossy green foliage and ruffled copper-orange petals. It has enticing yellow stamens that attract bees and other pollinators.

(Rita Perwich)

5) Roses need fertilizers high in phosphorus.

False. There is no scientific evidence that roses need high levels of phosphate. Phosphorus buildup is caused by extensive use of inorganic fertilizers. It is detrimental to plants and can result in leaf chlorosis — a yellowing of the leaf caused by a lack of chlorophyll — because it limits the uptake by the plant of other essential nutrients such as iron, manganese and zinc. Don’t use fertilizers with a phosphorus number higher than nitrogen or potassium, and don’t apply phosphorus as often as nitrogen.

6) We should add mycorrhizae to our soil.

False. Mycorrhizae fungi assist plants in accessing water and mineral nutrients. But in nutrient-rich and adequately watered soils, plants are less dependent on mycorrhizae, causing the fungi to remain inactive. Scientific studies have found no significant value in the addition of packaged mycorrhizae to well-fertilized and well-watered healthy soils, so their application is a waste of money and resources.

7) Create basal breaks by adding epsom salts to the soil.

False. Many rosarians add epsom salts (magnesium sulfate) to the soil in the belief that it will help create basal breaks (new canes) despite the absence of published, scientific research. The routine addition of epsom salts can lead to salt accumulation in the soil, which can lead to leaf tip dieback, marginal leaf chlorosis and necrosis (or burn).

8) A soil test is unnecessary.

False. The availability of nutrients to the plant is affected by the soil’s pH. An incorrect pH can prevent our plants from properly taking nutrients from even a nutrient-rich soil, so what you put in the ground is not necessarily what the rose will take up. If you think you have problems with nutrient deficiencies, get your soil tested. Roses grow best in a soil that tests between 6.0 and 7.0, with 6.5 being ideal. When the pH falls below 6.0, the soil will hold potassium, nitrogen and phosphorus molecules captive. When the pH level is higher than 7.5, many plants will struggle to get enough phosphorus, iron and manganese.

9) Rose replant disease doesn’t exist.

False. This has been a controversial subject because not all rosarians experience this problem in their gardens. In some gardens, when new roses are planted where old roses used to be, they often struggle to establish and may not grow as well as they would if they were grown in soil never planted with roses. This scenario occurs even when the soil in the hole is amended. When the rose is removed, another species of plant can grow well in the planting hole. In affected gardens, microscopic soil-dwelling nematodes that feed on and stunt root growth appear to be a contributing cause of replant disease.

10) There are fairies at the bottom of the garden.

True, and not just at the bottom of the garden! This may sound like myth, but beneficial insects including lacewings, lady beetles and the larvae of syrphid and tachinid flies are just like good fairies and eat a big share of damaging pests. Look out for them and protect them by avoiding pesticides, especially broad-spectrum pesticides.

One more True or False: The San Diego Rose Society’s 93rd annual Rose Show will take place on June 12 at the Marriott hotel in El Cajon.

True! For more information, see the San Diego Rose Society website at sandiegorosesociety.com.

Perwich is a member of the San Diego Rose Society, a Consulting Rosarian and a Master Gardener with UC Cooperative Extension.




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