As the weather warms and the soil dries, many of us have turned on our irrigation systems and are watering our gardens.
Irrigation schedules may range from a few times a week to only once a month. Plants that require little water are referred to as drought resistant, and gardens that feature them exclusively as xeriscapes.
Plants vary greatly in their ability to withstand or evade drought and many from dry climates have developed a number of morphological and physiological strategies to aid in survival.
Drought resistant plants, once established, may only need water once a week, or even once a month depending on plant type, the garden’s location, the weather, soil type, depth, exposure and the organic matter in the soil.
Plants take up water through root hairs, most of which are in the top 15 inches of soil. Plants from desert regions like cacti and succulents often have extensive, shallow, fibrous roots to capture water from light rainfalls.
Leaves are reduced to spines and water is stored in swollen leaves and stems. Spines don’t just function as deterrents to predators but serve to break up air currents and minimize water loss through transpiration, the evaporation of water across the leaf surface.
Hairs on plant leaves perform the same function as spines to break up air currents and limit water loss across leaves.
Some plants have large noticeable hairs such as clary sage, and others are covered with fine wool like lambsears, mulliens like Verbascum olympicum and V. bomyciferum, and French lavender (Lavandula dentata).
Annual plants from summer-dry climates avoid drought by germinating, growing, flowering and setting seed during the rainy season and dying at the end of it.
Seed reserves remain dormant until rains allow germination when suitable conditions return. Most California annual wildflowers are in this category. They germinate and grow during cool weather when soil is moist, and finish their lifecycles as temperatures warm and the soil dries.
Exceptions are the tarweeds (Hemezonia and Madia) that follow this pattern but remain small until the spring annuals die, then grow and flower when this competition for space is minimized.
Summer-dormant bulbs like daffodils and narcissus evade drought by developing large underground storage units (we call bulbs) during the rainy season to store water and carbohydrates. They weather the dry season in a dormant state.
Other examples are our many native bulbs like freesias, sparaxis, ixia, squill, tulips, grape hyacinth and crocus. Naked ladies grow abundant foliage during the rainy season that dies in summer. Flower stalks are sent up in summer using the large reserves contained in the huge bulbs.
Some plants have white or silver foliage to reflect light and heat. We often grow them for their strikingly colored foliage. Artemisia, lambsears, Calocephalus brownii, santalina, dustymiller, Russian sage, lavender, some milkweeds, and Teucrium fruticans have beautiful silver leaves.
Some plants like manzanitas, olives, many oaks, myrtle, Italian buckthorn, strawberry tree, California buckwheats (Eriogonum) and others have leathery or waxy leaves with the stomata (where respiration and water transpiration occurs) recessed in the bottom of the leaf in deep stomatic crypts. Recessing the stomata in deep crypts limits transpiration. Thick, leathery leaves help reduce water loss.