Plants, the Adaptable Survivors
Though plants have many ways to survive dry conditions, most, such as leaf texture, are outside of our control.
Roots and the soil surrounding them can be influenced in many ways if we know what they need. In general, for drought-tolerant plants this means plenty of air and a deep root-run.
Roots, the foundation
Roots are the survival system of plant, gathering water, minerals and nutrients. Genetics govern the type of root system a plant will develop, and drought-tolerant plants often use a double root system, fibrous near the surface plus a deeper taproot, to make the most of available water.
When the top layers of soil dry out, the roots in that space go dormant, leaving the deeper roots to keep growing. So the plant gradually develops a root system that is deep and effective as the top soil dries. The lesson here is to water deeply and infrequently.
Take a look at the photo above, an example of the root system of a Swiss Chard plant a little over three months after planting. Each of the squares represents one cubic foot, so the the bottom roots are six feet below the surface. No info I could find on perennial roots, but it's unlikely that they are much shallower. If you'd like to look at more vegetable root systems, go to - https://soilandhealth.org/wp-content/uploads/01aglibrary/010137veg.roots/010137toc.html
Helping roots to expand
You can help roots grow downward, rather than staying in the upper levels of soil, in a number of ways:
1) Start plants in the deepest containers you can find. Not broad, narrow. Tall, compostable coffee cups, "tree pots" that are 14" deep and just wide enough to hold a 4" potted plant, or hinged seedling starters called "Rootrainers." Using these, you'll force the roots down into cool, moist soil from the beginning.
2) Loosen the soil to a depth of 20" or more, often called "double digging," with any fertilizer or amendments added at least 6" below the top of the ground. Avoid shallow digging that leaves a surface layer of good soil. Stratified soil leads to shallow roots that resist reaching farther for nutrients.
3) Add bone meal to the lower part of the root run. Bone meal has about 15% phosphorus, nourishing for root growth and slowly available for long-term effects.
4) Add kelp meal or water with seaweed extract. Kelp meal is a good source of phosphorus and potassium as well as nitrogen and seaweed extract is often termed a bio-stimulant, having growth-promoting enzymes as well as the usual fertilizer component. Excellent for promoting root growth.
Natural soil and compaction
Most soil that has never been plowed or "improved" is full of bugs, worms, fungi, decaying stem and more. Aeration isn't a problem, even in fine-textured soil. "Empty soil" or ground without roots growing through it, can become compacted under its own weight. Most people think of compaction as something that happens when you walk on wet ground too much, or when you drive equipment over it. Yes, those are definite no-no's but compaction can occur in just one winter, if there's no fibrous matter or stones to resist the slump. Aeration is one reason why vegetable gardeners used to turn over the soil every year, not just adding organic matter
The lesson here is to make sure you have roots colonizing the whole soil area, perhaps by growing annual flowers or a cover crop, or be sure to plant in to gravelly, sandy soil. And this applies to all plants, not just drought-tolerant varieties. Air isn't talked about as much as fertilizer, but it has the same effect on plant growth.
Natural growth vs. nursery-grown
If you plant sunflowers or larkspur from seed, the roots are going to grow downward right away.. No restrictions on how deep they can grow. But, in general, we buy our plants at a nursery. They're grown and sold in pots and the roots get a bit cramped, especially since customers like well-grown, almost flower plants. But drought-tolerance needs deep roots. What can you do?
First, tease out the roots as much as possible and, if you lose some or need to cut some off, balance this by removing some of the lower leaves. Then use some of the suggestions from the first section here to encourage the roots to grow downward, not outward. "Tree pots" are excellent for encouraging roots to grow to their full length before you plant.