Answer: This tree is deciduous, meaning it will drop leaves and perhaps some smaller twigs/stem material when weather changes. It's not clear exactly what size you mean by "branches." Mimosa is cold hardy to zone 6 (minus 10 to zero degrees) so the drop to the mid 30s shouldn't be a factor. You might want to take the lights off, perhaps their weight and/or heat is a factor.
You didn't say anything about how you water, but we find that watering is usually the problem in the vast majority of cases, both under and overwatering. I'm going to assume that your trees are on some type of drip system, but if not, the info below applies to any water delivery system.
Running drip several times a day 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. I've included some info on watering below:
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.
Use the 1-2-3 Rule as an easy method to remember how much water to apply. Small plants with shallow root systems, such as perennials, veggies, herbs, cacti, succulents have roots that reach about 1 foot deep, so water needs to penetrate that far. When the top 1 inch of soil dries out, it's usually time to water again. Shrubs have root systems that are 2 feet deep so water needs to soak 2 feet deep. When the top 2 inches of soil dries out, it's time to water. Trees are 3 feet deep. As plants establish root systems, the time between waterings can be lengthened, but it is always essential to water to the same depth. So you are applying the same amount of water with each irrigation regardless of the time of year, but the frequency changes. As warm weather arrives, you need to water more frequently than during winter.
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, browning along leaf edges, and leaf drop. Deep watering?or leaching?prevents this by flushing the salts past the root zone. Always water slowly, deeply and as infrequently as possible. I hope this info helps.
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