Firstly, it is important to know that a completely unirrigated landscape is possible.
Not only that, but it can be very lush and colourful, and may be impossible to distinguish from an irrigated landscape.
What is special to unirrigated landscapes, and are there any disadvantages?
Here is what makes them different.
(other than the obvious):
-Different Plants. Some plants perform better without irrigation, others worse. Some cannot survive. This means no bananas, and in Grand Junction, it may even be too dry for Lavender, sadly. This is where it is important to know that there is a distinction between totally without water and a "dry" garden (a relative term) where you can still, in fact, grow Lavender. However, that list of plants which will, in fact, do it, is amazing and growing longer all the time. If your goal is to grow Giant Orienpet Lilies, then make a watered area for them and their other thirsty comrades. If you are hell-bent on growing certain plant, that's okay (I hope so- for it is my life story); what you need is more of a "garden." *
Sainfoin, or Onobrychis viciifolia requires no irrigation in a climate with 9 inches of annual rain and does quite fine in compacted clay. It also makes people pull over and ask what it is.
-There are fewer pests. Perhaps this will change, but I have observed that in a neighbourhood environment full of traditional landscape pests, natives have fewer. This may change in future years as folks grow more natives. Still, wild natural plants are more built to avoid, cope and recover from pests.
-New plants need water to get started, and also prefer to be bare-root planted.
-No Irrigation system or its interminable, abominable, maintenance. Just hose-dragging to get things started, then you're free.
-Rain is the irrigation. So plant placement is subservient to where rain naturally accumulates or does not. (I tend to use gravels and cactus in high, dry spots and shrubs or grass in low, moist spots; this also helps prevent weeds. Downspouts must be acknowledged and utilised )
-Fewer Weeds and Less Pruning. There are simply less resources for living plants in the area, so it limits weeds, which are nature's army meant to fill that niche and use those resources. Secondly, no-irrigation appropriate plants grow with their natural expectations, so giving them water simply encourages their bad behaviour. So spending more (water) on such plants results in spending more (time/money) to prune. Why do that?
-The Native tendency. It's no mystery that one's local flora will be the happiest plants with local climate, so I won't insult readers' intelligence explaining it. What we all must remember is that there are natives who aren't appropriate for our landscape: lovers of water (like willow and dogwood, which grow near water) and there are many many natives we don't encounter very often which will surprise us with their lushness, colour, and surpass our expectations.
Jones Bluestar, Amsonia jonesii, does not look like a plant tougher than cactus. But it is.
Blackfoot Daisy, which is essentially in continual bloom, may go without water here, but I'll wait to rule on that for another year or two.
-Unwatered landscapes need not be "wild." Going unwatered doesn't restrict what style the landscape has. Natives are not restricted to looking twiggy or unkempt. An fully-xeric native garden also does not necessarily need to be informal. It could be clean, formal, beautiful, or as boring as any traditional bank-parking lot landscape. (I made some like that early on- I'll admit. )
Clean lines and lots of green- Does this look like an all-native unirrigated Xeriscape? It is.
(Bouteloua gracilis, Blue Grama is the ornamental grass in this six-month old landscape)
* Landscapes v. Garden
Going 100% without water is more appropriate for Landscapes than Gardens. So let's distinguish them for argument's sake:
-A Landscape is a patch of earth we must deal with, so we make it pretty. We are forced to work on it. We have space: what to do with it?
- A Garden is a need to grow things, so we sought a patch of earth to do it with. We found a place to do our work. We have a need (veg, flowers): where's the space for it?
It's in our heads: One of the above can magically instantly morph into the other based solely on how a person feels that minute! Most of us deal with spaces which are sliding around on a spectrum in-between the two.
What I'd like to discuss, and invite other's revelations about, is the limit of unirrigated landscapes. I think they are few, but exist. An amusing flaw may be that a garden which requires little to no maintenance is easy to ignore, and we'll forget to enjoy it! (I also confess to this, with frequency)
It's easy to overlook new flowers, like Chamaechaenactis scaposa, (Penstemon arenicola in background) in a carefree unwatered garden that does not demand your attention.
But the real thing which is bad about the Unirrigated is on the extreme end of the spectrum: Garden. Since it serves our whims and joys, it is less adaptable to those needs if it is deprived entirely of the tool of irrigation. Not to say we can't be responsible resource users within that realm. My "guilty pleasure" garden, which, for the confessional record, hosts orchids, Gentians, and yes, a freaking Banana, is watered but once per week in the summer, and about fortnightly in the shoulder seasons of spring and fall. It uses a variety of mulches, topdressings, rocks, and ground-covers to retain that weekly water.
Traditional Veg just can't be grown here without irrigation. There are edible xerics- but grown dry, their production is unreliable and their space-to-food ratio is poor. (i.e. Clove Currant, Goji Berry, Saltbush, Lamb's Quarter, Wild Onion, Purslane, Desert Holly, etc). We must water if we want (realistic) food.
A freaking Banana in Grand Junction, Colorado. (planted 2005) Musa basjoo https://en.wikipedia.org/wiki/Musa_basjoo
Bletilla ochracea, Yellow Ground Orchid.
But the rest of our yards? The rest of our parking lots, parks, medians, lawns, verges, alleys, dog-runs and parkways?
Fair game for the unwatered approach, do you agree?