PPDL Picture of the Week for
July 30, 2012

Extreme Heat + Drought = Double Trouble

Rosie Lerner, Extension Consumer Horticulture Specialist and Master Gardener State Coordinator, Purdue University

Dry summers are not that unusual in the Midwest, but this year's dry spell - and now drought in many areas - has come considerably earlier than usual. Summer is just now officially getting started!

Gardeners have a battle on their hands to keep plants healthy when extremely high temperatures are accompanied by lack of rain. During prolonged drought conditions, water restrictions or just limited ability to water to a huge number of plants, you may have to limit watering and prioritize which plants will be rescued - a bit of garden triage, so to speak.

Newly planted trees, shrubs, flowers and vegetables will be at most risk of drought injury. Most gardeners are accustomed to watering flowerbeds and vegetable gardens on a regular basis. These plants require approximately 1-1 1/2 inches of water per week to maintain optimum flowers, foliage, roots and fruits. In times of drought, established plants may tolerate 10-14 days between watering, but be aware that problems such as fruit cracking and blossom end-rot will increase. Watering is most critical at pollination and fruit-set time for most vegetable crops, and we are right in the thick of fruit-set time in most vegetable crops. Use mulch where possible to conserve what moisture there is.

The best way to water is by slowly soaking the soil thoroughly in one application. This slow, deep watering will encourage deeper root growth, which in turn will be better able to withstand the drought. Frequent shallow watering encourages shallow roots, which are more likely to succumb to heat and drying of the topsoil. Sandy soil and containerized plants will need more frequent irrigation than a heavier clay soil or a loam that has good organic matter content.

Leaf scorch on trees and shrubs, appearing as a browning along the edges of the leaves, is very common in dry summers. While minor cases of leaf scorch are not very harmful, prolonged lack of moisture can spell disaster for landscape plants. Young and newly established plants are most susceptible to the dry conditions, but even established plants may reach a critical point during prolonged drought. Branch dieback, combined with eventual root death, will make plants more susceptible to winter injury.

Plants that were already under stress from other factors have likely already succumbed to severely dry soils and if the extreme drought continues into fall, we will no doubt continue to see more damage in the year to come. Next year's growth will be determined by buds that form this summer and early fall. Flower buds for spring flowering and fruiting plants will also be developing during this time. So the damage inflicted by drought now may affect next season.

Direct watering of landscape and fruit plants to where the roots naturally occur. While woody plants do have some roots that grow very deep, the feeder roots, which are responsible for most-efficient water uptake, occur in the top 12-18 inches of soil. They are concentrated below the dripline of the plant and beyond, rather than up close to the trunk. Apply water at a slow enough rate to allow penetration, rather than wasting water by runoff, and thoroughly soak the target area. Don't apply the water any faster than one inch per hour. As with annual plants, mulch will help prevent moisture loss due to evaporation.

Click image to enlarge

Arbovitae scorch and dieback

Scorch on flowering cherry

Scorch on flowering cherry buds

Hosta scorch

Scorch on oak

Oak leaf scorch

Ilex dieback



Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service