July in the Garden

by Wayne Myers, Master Rosarian, Jacksonville Rose Society

STAY SAFE: Severely hot weather and hurricane season are here. Our oppressive heat and afternoon showers can be daunting. Try to avoid working in the sun during the hottest part of the day. Accomplish your rose chores and enjoy your flowers in the morning or in the evening. I congratulate Ed Buck—he sprays at night to save the bees and endure less heat stress. The days are so long that there’s plenty of time early and late for rose care without risking heat injury. Even so, wear a wide-brimmed hat and sunscreen when you’re out in the sun. For years I’ve also worn gloves, long sleeves, & long pants. Remember to HYDRATE! ABOVE: Keep your beds well-mulched, photo by Rita Perwich.

Just as we gardeners need lots of water to stay safe and thrive in the heat, our roses must have plenty of water to prevent heat damage and continue flowering. On dry days, late-afternoon drooping of leaves and new growth is the plant’s natural reaction to excessive heat. All we can do is keep our plants healthy, keep the beds well-mulched, and water them unless it rains. By late June most rose-gardeners decrease feeding as day temperatures climb the 90’s and nights rarely drop below 80.

DEADHEAD: As your blooms expire, remember to deadhead. Dead-heading regularly will encourage more flowers. Dead-heading can be as simple as snapping off the spent blooms whenever you see them, or it can be a well-thought-out strategy to keep your plants healthy and restrained. Bushes that have lots of healthy foliage are the key to survival in summer’s heat and extreme disease pressure. So if your bushes are healthy, but bigger than normal or desired, don’t hesitate to “deadhead” back to the first five-leaflet leaf—or further—if you need to control the size of the bushes. Remember, though, that temperatures in the 90s really stress our plants and we can expect intense black spot pressure, chilli thrips, and spider mites.

Here is an explanation of summer deadheading by Ludwig Taschner, the “Rose Guru” of South Africa:

‘Breaking or cutting off the dead or faded bloom above the upper most leaves results in new growth sprouting from the highest leaf axils. Expect many short-stemmed smallish blooms within 30 days. By cutting the stem about half-way down on the overall stem length, two medium long-stemmed blooms are ready after about 38 days. Cutting a long stemmed bloom above the second leaf, counting from where the stem sprouted, results in one long stem after 45 days. All this may be carried out on each bush. The very long-stemmed varieties can be cut back by about half to avoid them growing too tall.’ (“Talking Roses with Ludwig,” Nov 2010)

Ludwig’s rose advice is six-month out of synch with our northern hemisphere climate, but his rose wisdom is unparalleled. Check out his web site at www.ludwigsroses.co.za/.

SUMMER DORMANCY: Geoff Coolidge of Cool Roses, West Palm Beach, Florida, uses a contrarian approach to hot weather rose care. Because many of his rose-care clients are not in Florida during our worst heat and disease pressure, he encourages his clients’ roses to go dormant in the heat of summer. By not deadheading and feeding, he discourages new growth which is more vulnerable to pests and disease than old growth.

Most rose bushes grow much more slowly when daytime temperatures are over 90. Furthermore, conditions for chilli thrips, spider mites, and black spot are ideal when humidity is high and night-time temperatures stay above 80. Mature rose foliage is much more resistant to disease and pests than new growth. Leaving spent blooms on the bush, rather than deadheading, causes the plants to redirect their energy to producing seeds, rather than creating new growth and more blooms.

Although our North Florida climate is cooler overall, I agree with the plant–science behind Geoff’s encouragement of summer dormancy to discourage the summer scourges of black spot, chilli thrips, and spider mites. However, I don’t understand, why any hobby rosarian would undertake the trouble of rose care through the summer without the payoff of flowers—even though they are fewer, smaller, and paler.

BLACK SPOT: This fungal scourge affects only roses. Florida gardeners must either grow leaf-spot-disease-resistant varieties or maintain a spray program that includes both contact and systemic fungicides. Ideal conditions exist for spore germination and fungal growth when temperatures above 70˚ and moisture is present for 6-8 hours—doesn’t that sound like EVERY Florida night from late May till late September? Like other disease pathogens, fungi can evolve to resist systemic chemicals, so it’s best to alternate the systemic chemicals in your spray mix. When I sprayed, I alternated the Banner Maxx generic Honor Guard (Qual-Pro is a cheaper generic of Honor Guard) and Cleary’s 3336F because they are both effective, have different Modes of Action (MOA), and are easy to acquire and legal to use in FL. Each spray mix should also include a contact fungicide such as Dithane M-45. Spraying every other week will prevent black spot on most varieties. Don’t forget to add a spreader-sticker—Miller’s Spray-Aid and Scarlet are effective. ABOVE: 'Knock Out' black spot. BELOW: 'Rose de Rescht' black spot

Keep up your spray program—if black spot establishes, you must at least double the spray frequency until your foliage is again clean. Some rosarians will strip off any leaflet showing black spot, and pick up and remove any diseased leaves that have fallen to the ground. As black spot progresses in a leaflet, the leaflet turns yellow and drops to the ground. The infectious black spot fungal spores are scattered from the black spot lesions and most often spread by water droplets.

Again, I recommend the booklet “Guide to Rose diseases and Their Management” (RIGHT) by Drs Mark and Alan Windham and Alan Henn. It’s a free download from the American Rose Society’s website at https://www.rose.org/diseases.

CHILLI THRIPS (Scirtothrips dorsalis): For more information about Chilli thrips, review the information on the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Chilli thrips web site at http://mrec.ifas.ufl.edu/lso/thripslinks.htm. The site has lots of pictures of the critters as well as pictures of their damage on roses and other plants. The pictures of their damage are most helpful because chilli thrips are so small that they can’t be identified by the naked eye.

In 2009 through 2011 I successfully prevented major chilli damage in my garden for three years by applying granular Merit (imidacloprid) in late March. However, in 2012 and 2013 I lost many more bushes to chilli thrips. The adult chilli thrips is approximately ½ the length of the Western Flower Thrip (WFT). Chilli thrips favor new growth over the buds and blooms favored by the WFT. Deformed, crinkly new shoots and leaves are the earliest signs of chilli thrips damage that appear before you will see damaged blooms. As damaged leaflets mature, they will have brown or gray scars on their undersides. However, severe, untreated infestations will also result in severe bloom damage and possibly death of the plant. ABOVE: chilli thrips, photo credit University of Florida

To see them, tap a badly distorted sprig of new growth on a piece of white paper, or swish it in alcohol. Any small specks like pepper on the paper or in the alcohol could be chilli thrips; confirm by looking with a strong magnifying glass or jeweler’s loupe. The mrec.ifis.ufl web site also lists several systemic chemicals that are effective against chilli thrips. After going to the home page of the chilli thrips site, click on the “Management” link to see the list of chemicals.

An untreated chilli thrips infestation kills most varieties of rose bushes, or weakens them so much that they die from black spot and heat stress or are permanently stunted. University of Florida research found that on susceptible rose varieties, cultural and biological control measures could not control chilli thrips; therefore, to control them requires spraying and/or drenching with a systemic insecticide rated for thrips. Dr. Vivek Kumar, a thrips researcher who worked under contract for the US Department of Agriculture and Florida IFAS, believes that drenching the soil around roses with imidacloprid or cyantraniliprole is effective and would be less damaging to thrips predators and pollinators than foliar sprays.

In my own gardens, certain varieties seem less susceptible: ‘Napoleon,’ ‘Smith’s Parish,’ Rosa rugosa alba,’ ‘Mrs. B.R. Cant,’ and most of my Hybrid Musks show little chilli thrips damage when others must be sprayed or die. My Kordes hybrid teas ‘Beverly’ and ‘Wedding Bells’ also seem resistant. Left unsprayed, chilli thrips killed my ‘Knock Out’ roses within three years. These observations that rose varieties vary in susceptibility to chilli thrips were confirmed by the small UF/IFAS research project for which the Jacksonville Rose Society provided supplemental funding in 2016.

Systemic insecticides, such as Merit (imidacloprid), Conserve (spinosad), or Orthene (acephate), provided effective control in UF/IFAS trials. These three chemicals have different Modes of Action (MOAs). Never use a systemic pesticide with the same MOA more than twice in a row. You may have to spray as often as every two weeks, rotating chemicals by MOA. For even better control, as UF/IFAS scientists determined, drench your rose beds with a systemic in addition to spraying. Drenching my rose beds with Flagship (thiamethoxam) provided good control in 2014. The manufacturer claims eight weeks of control after a drench and says the chemical is gentle to beneficials.

According to Gaye Hammond of the Houston Rose Society, rosarians there have been able to control chilli thrips with Suffoil-X (a spray oil emulsion insecticide), but Florida researchers found it to be only marginally effective in their research. If you spray for chilli thrips, avoid the middle of the day. Around mid-day, the adults fly in swarms and spraying would miss many of them. Spray very early or very late to minimize danger to pollinators. If chilli thrips are causing unacceptable damage to your roses, you will not be able to eradicate them—only suppress and control them.

Chilli thrips are opportunistic, polyphagous (they will feed on many, many different plants) feeders; therefore, if they are on your roses, they are also established on other plants in your landscape. Just before I sprayed my new St. Augustine roses for the first time to control chilli thrips, I had pruned the ligustrum hedge along the west side of the house two weeks earlier. To my dismay, the emerging new growth on the ligustrum showed chilli thrips damage almost immediately. University of Florida scientists have identified over 100 economically significant plant species that chilli thrips will damage. See the list at http://mrec.ifas.ufl.edu/lso/thripslinks.htm#HOSTS.

Although it has not been confirmed by scientific research, another valuable observation from Geoff Coolidge of Cool Roses, West Palm Beach, is that he suspects hot nights are more important than hot days to increasing the reproduction rates of chilli thrips. Their reproduction and damage may also be slowed by spraying with a water wand as you would for spider mites. Spray the entire plant, but remember that chilli thrips prefer the new foliage.

SPIDER MITES: Spider mites are another hot weather pest that can devastate your roses. Evidence of mite damage usually appears first on the plants’ lower leaves because the mites’ reproductive cycle takes place in the soil. The adults climb up the bush to feed, suck juice from the leaves, then fall back to the ground to reproduce. Watch for dried and browning leaves that have spider-like-webbing and fine-sand-grainy-looking residue on their undersides. Some rose varieties are much more susceptible than others—spider mites showed up first on ‘Don Juan’ and ‘Rose de Rescht’ in my garden.

Don’t ignore them--an infestation left unchecked can severely weaken a plant. Normal insecticides are not very effective, but there are specialty chemicals called miticides (acaricides) available. However, the most effective are hard to find, expensive, and illegal for home-landscape use. The best, Avid (abamectin) is also effective against chilli thrips, but may even damage your plants if applied in hot weather.

Fortunately, water spray can provide adequate control. Use a nozzle to forcefully wash your bushes from the bottom up, concentrating on the undersides of the leaves. My spider-mite wand works well. Because the spider mite’s life cycle in hot weather is approximately seven days, wash the plants again four or five days later, then again after another four or five days. Your heat-stressed bushes will benefit from the extra water and the natural predators—stronger and more mobile than the mites—will return to finish off the mites that weren’t blasted off the plants by your spray. ABOVE: Spidermite and lacewing egg.

MULCH: Beyond frequent watering, how can we help our roses endure the heat? Mulch would probably top any list. Mulch shades the beds, keeps soil temperatures lower and more stable, and decreases water loss through evaporation. Surely you’ve noticed that every season brings new varieties of weeds that will out-compete our roses? These seasonally-optimized weeds easily out-compete our roses for nutrients, and blemish the appearance of our gardens. Discourage their germination by thickly mulching your beds. Several years ago, I had such thick Purple Nut Sedge and Florida Betony that I gave up and treated the worst beds with Roundup—repeatedly—before I could re-use the beds.

FOUR-LEGGED PESTS: Armadillos have contributed mightily to my weed woes. Healthy rose beds have lots of earthworms; a favorite armadillo food. All spring they dig with their snouts for worms. The “nose holes” they leave in the mulch covering the beds quickly fill up with weeds. The bare spot can’t simply be covered over because the creatures have compressed the pine straw and created mounds and holes that have to be carefully smoothed and rearranged unless you re-mulch frequently to cover their bare spots.

If deer are browsing on your roses, spray with Liquid Fence from Tractor Supply or try one of the home brews at http://www.deer-departed.com/deer-repellent.html The “secret” solution recommended by one of our members is to liberally sprinkle human urine in your rose beds.

From personal experience, I know the commercial repellent does not have to be renewed as often as pee. In his Wisconsin garden, Will Radler, the hybridizer of ‘Knock Out’ roses, distributes small (1/2-cup) piles of milorganite every ten feet along the perimeter of his beds to repel deer. Renew the milorganite every couple of weeks. This method has worked for me in St Augustine. It should work well unless your garden is already a part of the local herd’s nightly browsing route. I’ve even noticed un-nibbled roses when fresh deer scat was immediately outside my Milorganite barrier.

This year I forgot to lay out my milorganite “fence.” Therefore, in late March, the deer grazed all my front yard rose bed. I laid out a perimeter of milorganite, and they haven’t been back.

FERTILIZER: To feed your roses through the summer heat, I recommend time-release fertilizer (heat release; NOT water release) like Osmocote and organics such as Jim Young’s Purely Organic or Mills’ Magic. Both types release the nutrients slowly as the soil microbes break down the organics and resist the leaching that naturally occurs in sandy soils because we must water frequently, and thunderstorms provide drenching downpours.

Enjoy and share your roses!

Photos by Wayne Myers unless specified otherwise.