Summertime Rose Dangers

by Carolyn Elgar, Master Rosarian, Orange County Rose Society

This is a 2020 AOM winner

The spring, with its first flush, is the optimum time for rose blooms. Cool nights, mild days, lots of sun, not too much rain - it’s a great time for gardeners. But roses don’t go away after the spring turns to summer. Unfortunately, a lot of us do, either vacationing or staying inside because of the heat; it’s not fun to work in the garden when you are covered in sweat and invaded by insects that get in your eyes or bite you. The hot days of summer can be a dangerous time for rose plants. There are things out there that will kill your roses during the summer, if you aren’t careful.

Lack of water

Roses need water. Water is a valuable commodity in many places, especially hot ones. But rose gardens are not drought resistant. If your rose plant does not get enough water, it will weaken. Too many days of not enough water can kill a plant, especially if it’s a small one or in a pot.

In the cool days of spring, it’s easy to catch up on watering if your system fails. Lower temperatures give the roses some time to wait between waterings. But in the heat of summer, especially the extreme heat so many of us are experiencing as our climate changes, lack of water has a quick impact on rose health.

Gardeners need to check the efficiency of their watering systems on a regular, if not daily, basis during the heat of the late summer months. Hoses can become clogged, animals can chew up drip rings, or you may be still watering on a spring schedule. It is important to check the soil for moisture. You can use a soil probe or a stick, but you must get below the surface of the soil to truly judge how dry it is. If the surface is dry, but the soil is damp beneath, the situation is not critical. If the soil is dry under the surface, you must get water to the plant immediately. Roses’ roots tend to be shallow, from 12 to 15 inches deep, and the largest amount of them are in the upper third of the root zone. Make sure you check the soil to that depth.

The quality of your soil has an influence on how water is absorbed. Soils contain sand, clay, and silt. You can find out how much of each is in your soil by doing a jar test. Collect a cup of soil from the garden, strain out the debris, and put it in a glass jar. Add water until the jar is around 3/4 full. A few drops of dish soap will help break up the water tension in the jar. Shake the jar for one to three minutes and leave it along for a day. When you check the jar, the sand will be the heaviest, at the bottom of the jar, the silt will rest above the sand, and the clay will create a layer on the top. You can actually measure the percentage of each by measuring the total level of the soil and the different layers in the jar. Divide the measurement of each layer of sand, loam, or clay by the total amount of the soil.

All three components of the soil are important, but large quantities of clay or sand will impact the amount of water your plants’ roots will absorb. Sandy soils will drain quickly, while clay soils may have to be watered more than once because water penetrates clay slowly. In sandy soil, one inch of water will penetrate to 12 inches. Its field capacity (the amount of water left in the soil after irrigation, the water uptake, and runoff) is around seven percent due to quick drainage. An inch of water penetrates clay soil to around four to six inches, but because it drains slowly, 35% of it remains in the soil.

The ideal soil for roses is a loamy soil that consists of less than 50 percent sand, from 10 to 30 percent clay, and between 30 and 50 percent silt. Water will penetrate this soil at around six to ten inches; loamy soil will have a field capacity of 25 percent.

Adding organic matter to a sandy or clay soil will increase the loam amount.

Knowing the amounts of the major components of your soil will guide your watering activities. If you have a lot of sand, you know you will have to water more often. Lots of clay impedes drainage, so you will need to check soil moisture before you water.

Potted roses are very susceptible to drying out in the summer heat. It is also easy to miss any watering problems with pots, especially if they are on drip watering. A rose in a pot does not have the buffer of cool, deep soil; it is stuck with the pot and its soil. More than one rose lover has ventured outside in hot weather and discovered a plant with yellow leaves, struggling with dry potting soil.

If the soil in a pot has dried out too much, it may not absorb water anymore. If the soil has pulled away from the edges of the pot, water may just run down the inside and out the bottom. Dried peat in potting soil is challenging; it is hard to rewet and may actually repel water.

One thing that will help revive a dried-out pot is submerging the entire pot in a larger pot of water and let it sit for a while. Air bubbles will be released from the soil as they are displaced by the water. When the bubbles are gone and the pot that was dried out is heavy, take it out of the water. Or set the dried-out pot in a shallow container of water and allow time for the soil to slowly absorb the water, maybe an hour or more. If a pot is too large or heavy, put a hose that is trickling water on top of the soil for an hour or so; a dried-out soil needs time to absorb water.

ABOVE & BELOW: Heat stress shows as brown and yellow scorched leaves. Potted roses can become stressed quickly when not watered.

Heat stress

Roses love sun, but they don’t like temperatures over 85 degrees. High temperatures affect the major plant growth processes of photosynthesis, respiration, and transpiration. Photosynthesis is the process of converting light energy to plant sugars. Respiration is the process in which the plant uses those sugars for energy that supports plant growth and development. Transpiration is the process of water moving through the plant, distributing nutrients, and evaporating through the openings in the leaves.

At high temperatures of 95 degrees and above, the enzymes that are responsible for fixing carbon in photosynthesis become denatured, losing their shape and function. Thus photosynthesis slows or stops. However, plant respiration continues, using up available plant sugars. Without that energy, plant growth slows, leaves weaken, and blooms become smaller.

In transpiration heat speeds up the evaporation from the leaves, increasing the need for water in plant tissues. Openings in the leaves called stomata are where the evaporation occurs; this evaporation is how most of the water in the plant is eventually lost, leaving only 25 percent in the plant. The stomata have guard cells that open and close them. These cells need to be turgid with water in order to function. When they lose their rigidity because of the rapid use of water, they can no longer do their jobs and the stomata close. The leaves are no longer cooled by the evaporation; they begin to scorch and burn. The plant reserves what water it has for the new, young leaves, so heat stress shows up first on the older, upper leaves as yellow and brown scorching.

Consistent high temperatures lead to depletion of plant sugars and the diminishing of water movement and evaporation. Research has shown that flowering plants are most affected by heat stress in the two weeks before they bloom. Blooming is an energy intensive activity that plant sugars and fluids support; as a result, blooms are smaller or even fall off the bush when plant processes cannot provide what they need.

Different roses have different tolerances for heat. As you get to know the roses in your garden, you will learn which ones are less tolerant. For example, Sweet Chariot, a miniature rose, does not tolerate heat well and will yellow and shrivel in it. The roses you know are susceptible to heat stress need to be shaded or moved to a shady place. For a list of roses that can tolerate the heat, go to the Pacific Southwest District website and look under Rose Info. Roses are listed by class.

In very hot temperatures many rosarians who live in desert areas use shade cloth to protect their plants. If you only have a few plants to be shaded, you can use a light-colored umbrella, an umbrella covered in foil, or move the roses to a patio. If high heat is an annual occurrence and you have a lot of roses, you may want to consider constructing a shaded area using shade cloth and pvc piping.

A layer of mulch helps keep roots cool and retains water around the plants. It also helps suppress weeds. As the summer progresses, give your roses a daily morning spray of water. This will wash the dust and insects off the leaves and provide some relief to the water starved leaves. Don’t cut off the scorched or burned leaves as they provide shade for the newer leaves below them.

ABOVE & BELOW: Chilli thrips’ damage shows as long, spindly, deformed canes and leaves. Cutting infected new growth is a necessity.

Insect Invasion

Warm temperatures make your garden hospitable to lots of insects. Two of them can destroy a rose. Chilli thrips and spider mites both increase reproduction greatly during hot weather. Both are very small; only their damage is visible.

Chilli thrips attack the new fresh growth on the top of the plant. Leaves are distorted and crinkled, curling inward and becoming brittle. Buds become discolored, dried out and shriveled. New stems are left bare as leaves die. These tiny creatures move by air currents and can easily spread in the garden. Because of this, and the degree of their reproduction, chilli thrips must be dealt with as soon as there is evidence of them.

The extreme damage chilli thrips cause had led to lots of research on what works best to get rid of them. Sadly, for organic gardeners, spraying is a necessity in dealing with these pests. Cut out the affected areas and spray immediately with spinosad, that has, so far, proved to be the most effective measure. Spinosad is available locally as Monterey Garden Insect Spray or Captain Jack’s Deadbug. A more expensive and more effective spray, Conserve, is available online. When the weather turns warm, many rosarians include spinosad with other things they spray, such as fungicide; it can be tank mixed. Typically we spray for insects when we see them or their damage, but chilli thrip is so invasive that spraying preventively is frequently done.

ABOVE & BELOW: Bronzed, dirty looking leaves and webbing between leaves are signs of spider mite infestation.

Spider mites are similar to chilli thrips in that they rasp leaves, leaving them brittle and crispy. They occur first on the bottom part of the plant and move upward; they live in colonies under the leaves. Leaves with a bronze cast or specks of black (excrement) are signs of spider mite infestation. Like chilli thrips they spread quickly, from plant to plant. Remove damaged growth. Hard sprays of water aimed at the underside and tops of leaves may kill them, but this water spraying should be repeated every three days so newly hatched mites are addressed. There are products that will kill spider mites, but they will also kill any of the beneficial mites that eat insect pests. The products are expensive and their consistent use can lead to mite resistance. Because of this, perhaps the best recommendation in the case of a bad infestation is to use a miticide, such as Avid, as the first step and follow up with repeated water sprays.

There will be plenty of other insects, such as grasshoppers, flower thrips, and rose slugs, in your garden during the summer. They may cause unsightly damage, but they won’t defoliate or kill a bush like chilli thrips or spider mites can.

These are the top three areas of concern for your rose garden during the hot days of summer. All three will have a great impact on your plants unless you stay on top of them. Even though it’s hot, walk the garden daily and look for problems. Plant damage happens faster in the heat, and the roses can’t wait. They need your care.