One of the most confusing terms used in the gardening world is “drought tolerance.” As water becomes both scarcer and more expensive, more plants in garden centers bear this designation, sometimes accompanied by the words “once established.”
Our summers are hot and humid, but the real killer is our high nighttime temperature, forcing our plants to work through the night. Notice when nighttime temperature falls into the 50°s how refreshed our plants suddenly look. If new plants—and by new I mean those plants we planted in the spring or previous fall—are to work through our hot summer temperatures, they need help as otherwise they will become exhausted.
Many perennials frustrate the new gardener because for the first season they appear to be just sitting, doing nothing. However, beneath the soil line there is a lot of activity occurring as the roots are growing and getting established in their new home.
Roots do far more than just anchor the plant. They supply the plant with water and nutrients while the leaves, through photosynthesis, provide the roots with energy: think of the stem as a two lane highway, one lane sending water and nutrients up to the leaves, the other lane sending energy down to the roots.
While the roots are getting established, they need some help in supplying the plant with water. Watering plants can be a tricky thing: too much water will deprive the roots of oxygen and the plant will die and too little water will cause the plant to die from thirst.
There is a right way and a wrong way to water plants. I shudder when I see overhead sprinklers in use in a flowerbed because the water needs to get to the roots, not the leaves. Wet foliage encourages fungal disease, so it’s important to water in the morning so the foliage can dry off. Water should be directed to the roots system either by hand watering or by using a drip system. Remember that the leaves will derive water via their root system, not through their stoma on the leaves.
All this gets me back to the term “drought tolerant.” Many plants, such as baptisia, echinacea, sedum, and ornamental grasses are truly drought tolerant once established. The period of establishment is different for every plant so it’s important to research your plant. Ornamental grasses need a good source of water for only four weeks because they root quickly. Echinacea takes a longer time to get truly established and will require water throughout that first summer. Baptisia takes one or two years, putting all its energy into establishing a taproot before top growth becomes apparent.
The important thing to remember when buying a “drought tolerant” plant is that it will need some TLC before it actually becomes drought tolerant.
Kit Flynn is a Durham Master Gardener and a member of the Advisory Committee of the Durham Garden Forum.