A drought is atemporary reduction in water or moisture below the expected or normal amounts for some time. A drought can be short, lasting a few weeks to one or two months, or long, lasting for years.The occurrence of drought has become common today as we witness incidences of severe weather events related to global climate change.

What is the impact of drought on vegetation?

Plants rely on moisture to thrive and be productive. When moisture is inadequate, plants are stressed and productivity is reduced.Plants go through a ‘shutting down process’ when they begin experiencing moisture stress, including decreased leaf size, dropped stomatal number, thickened cell walls, and reduced leaf surface.

Classification of drought

The United States National Drought Mitigation Center (USDMC) monitors drought around the country and developed a system to categorizedrought severity. The system is used to help determine the need for drought mitigation strategiesto provide relief for those who may be affected by the drought.

The USDMC categories are:

D0: Abnormally Dry

• Characterized by short-term dryness that can slowplanting operations and the growth of crops or pastures. During this type of drought, there are usually some lingering water deficit issues and pastures or crop do not fully recover when watering regimes return to normal.

• Each year, 53.3% of the United States is characterized by this type of drought.

D1: Moderate Drought

• Characterized by some damage to crops and pastures. Streams, reservoirs, and wells have low water levels and voluntary water-use restrictions are requested

• Each year, 36.9% of the United States is characterized by this type of drought.

D2: Severe Drought

• Characterized by the likelihood of crop or pasture loss;water shortages are common. During this type of drought, water restrictions are usually imposed.
• Each year, 25.9% of the United States is characterized by this type of drought.

D3: Extreme Drought

• Characterized by major crop or pasture losses and widespread water shortages or restrictions.
• Each year, 18.3% of the United States is characterized by this type of drought.

D4: Exceptional Drought

• Characterized by exceptional and widespread crop and pasture losses. This type of drought creates shortages of water in reservoirs, streams, and wells, and itcreates water emergencies.
• Each year, 8.1% of the United States is characterized by this type of drought.

How do plants respond to drought stress?

When drought occurs, plants essentially go into ‘survival mode’ by engagingseveralprocesses that enhance their ability to withstand the period of drought stress. These include:

1. Increased root development – by having a deep and extensive root system, plants can draw water from deep within the soil profile. This is a characteristic of severalweed species that do well during periods of drought stress.
2. Stomatal closure – to prevent moisture loss through stomates.
3. Thickened cuticle – plants develop thicker cuticles, the waxy covering of plant leaf surfaces. This adaptation to drought stress helps the plant conserve moisture.
4. Hormone production – to help regulate processes within the plant that conserve moisture, e.g., stomatal closure.
5. Reduced photosynthesis – in the absence of water photosynthesis is limited.

Effect of drought on vegetation management

Herbicides are the most important tool used for vegetation management.  If herbicides are applied to moisture-stressed plants, they may not work as labeled. Why?

• For herbicides to be effective, targeted plants need to be actively growing which means that they have adequate moisture.
• Drought-stressed plants have thicker cuticles than well-watered plants. Plants with thick cuticles are hard to control because it is more difficult for the herbicide to be absorbed into the leaf.
• During periods of drought, plant leaf surfaces can accumulate dust or organic soil particles which can bind with some herbicides and reduce their activity.

How to overcome drought stress and control unwanted vegetation

Control of unwanted vegetation during periods of drought can be accomplished if conditions that improve herbicide uptake and control are addressed. For example:

• To address moisture stress,water the site that will be treated with herbicide a few days before application. This ensures that previously unwanted plants will be growing and can take up the herbicidewhen it is applied.
• Use overhead sprinklers to rinse the dust on plants. This helps ensure that the herbicides do not bind with dust or organic soil particles that would otherwise reduce their effectiveness.
• Use surfactant if permitted by the herbicide label. Surfactants are products that enhance the ability of an herbicide to:
  • Penetrate and enter into a leaf.
  • Remain in an aqueous solution on the leaf surface.
  • Reduce spray droplet surface tension and cause greater contact between the droplet and leaf surface.
  • Slow the evaporation of herbicide spray droplets.

Further Reading

• Brewer 90-10 Surfactant for Excellent Vegetation Management
• Adjuvants 101: Understanding What is in Your Tank
• Use of Silicone Adjuvant in Agriculture
• Cide-Kick II for Excellent Management of Weedy and Invasive Plants
• How Surfactants Affect Spray Droplet Size
• Postemergence Spraying and Tips for Application
• What is Methylated Seed Oil and How Does it Work?
• Adjuvants 101: Nonionic Surfactants
• Pesticides 101: Introduction to Pesticide Adjuvants and Surfactants
• Everything You Need to Know About Agricultural Wetting Agents

References

• Roncoroni, J.A. and C.A. Wilen. Managing Weeds Under Drought Conditions. Univ. California Agric. & Nat. Res. Green Bull. Vol. 4, No. 3, Aug. 2014. (Accessed, August 26, 2022)