Mimosa leaves wilting and rose leaves turning yellow - Knowledgebase Question

Las Vegas, NV
Question by lue5008
June 7, 2006
We have just bought a new home with 6 Mimosa trees about 12' tall on the west side. Just one is now starting to curl up some leaves and drop them off at an alarming rate. I also have roses all around the house, mostly florabunda type roses, and now their leaves are turning yellow. I have sprayed a pesticide, as I've seen some grasshoppers, and also some round shaped spots that are eaten on the rose leaves. I'm not sure what to do to get the roses back to blooming, and whether I can save the Mimosa.

Answer from NGA
June 7, 2006


Mimosa leaves curling and dropping and rose leaves yellowing are probably both related to ineffective watering. That is the most common cause of problems that we see. I've included some basics on watering and yellow leaves below.

Most people have drip systems but don't run them effectively. Running drip several times a day or for short periods is not effective because the root ball doesn't get moistened. For example, an emitter that puts out one gallon per hour would only put a quart of water on the ground in 15 minutes. For mature trees, water should soak 3 feet deep; for newly planted trees, about 2 to 2.5, depending on the size/depth of the rootball when it was planted.  For roses, water should soak 2 feet deep.

Use a soil probe (any long, pointed piece of metal or wood to poke into the soil) to check how far water has penetrated.  The probe moves easily through moist soil, but stops when it hits hard dry soil.  There are numerous variables involved for watering schedules, such as type of soil, how fast or slow it drains, sun and wind exposure at your site, temperature, age and condition of the plants and much more.  Use the information above to determine how moist the soil is before automatically applying more water.  It's essential that you allow your drip system to run long enough for water to penetrate the appropriate depth. Depending on the size emitters, soil type, etc. this might take several or many hours. If you use another system or even a hose, the information still applies.

Desert soil and water both contain salts, which can accumulate in the root zone over time.  This salt buildup forms where the water stops penetrating.  Short periods of watering cause salts to build up in the top layers of soil and damage or kill your plant.  Salt burn shows up as yellowing. Deep watering or leaching prevents this by flushing the salts past the root zone.  Always water slowly, deeply and as infrequently as possible.

To sum, insufficient water causes yellowing, and because salts also buildup in the root zone,  that causes yellowing. As a tree grows, its new roots tips, where nutrients are being absorbed, spread out laterally.  Expand your watering zone out PAST the tree's canopy as it grows.  As the tree grows, continue expanding that water zone.  If you have an irrigation system, you need to move the emitters out.  If you use a hose, just drag it out further.  In any case, water slowly and deeply to ensure water penetration and to leach salts below the root zone.

Yellow leaves can be caused by many things including lack of nitrogen, insufficient light, water-logged soil (plant roots need oxygen to thrive), dry soil, or iron deficiency. If the older bottom leaves are yellow, but new growth is green, it's usually a lack of nitrogen. If new leaves are yellow, with green veins, it's usually a lack of iron. Soil should be kept moderately moist (but not wet).  Finally, transplant shock can contribute to yellowing.  If new growth shows up as green, that might be the problem.  Try to isolate each of these possibilities one at a time to determine the problem.

Iron chlorosis is a problem in the desert for non-native plants (like roses and mimosa).  Chlorosis is recognized by new leaves that are yellow, while the veins remain green.  If the condition is severe, the entire leaf may be yellow. Although iron is present in the soil, it is not always in a form that non-native plants can use. (Native plants seldom suffer from iron chlorosis.) Overly wet soils are depleted of oxygen.  (As water fills in the minute spaces between soil particles, air moves out.)  Plant roots need oxygen to absorb iron in the soil.  To help prevent chlorosis, always water slowly, deeply and infrequently.

Soil with a high pH (alkalinity) also inhibits iron absorption.  If you are using correct irrigation methods and iron deficiency symptoms are still present, apply iron chelates or ferrous sulphate to the soil.  Both are readily absorbed by a plant's roots.

I would not recommend fertilizing with nitrogen now in the midst of summer. It stresses the plants and can burn roots, and often shows up as yellowing. Fertilize in early spring or fall just as temperatures start to cool.

Female leafcutter bees are responsible for those half-moon shapes neatly cut from leaf or petal edges. Thin, smooth surfaces such as rose petals and bougainvillea leaves seem to be favorite choices. A bee finds a hole slightly larger than herself, often in the ground or wood. She inserts several curled pieces of plant material to form a protective cell. Next, she fills the cell with a mixture of pollen and nectar to nourish her hatching larvae. Finally, she lays one egg, seals the cell with more plant material and is ready to repeat the process. Leafcutters are 1/4 to 1/2 inch long, black or grey and resemble honeybees. They are solitary creatures and ignore humans. The bits of foliage they remove do not harm healthy plants so no control is needed. If you plan to exhibit in rose shows, cover bushes with floating row cover. Do not spray. The bees do not ingest the leaf material so it does not good. Nor do we want to kill pollinating insects, or we'll have no fruits or vegetables! As for the grasshoppers, if there numbers aren't approaching a scourge, let the birds eat them. Spraying pesticides indiscriminately does more harm than good. Most insects are seasonal and will disappear when conditions change. Birds and lizards will strike a balance and consume all of your pests for you given the opportunity!

I hope this info helps. Good luck with your plants.

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