Watering your new plant is the real trick. We at D:FR feel that there is one common misconception widely promoted in the nursery trade with respect to planting cacti and succulents, and that is the notion that newly installed desert plants should be left in their freshly dug hole for at least a month, if not longer, without watering. The theory, we suppose, is that the plants will put out new roots and avoid rotting this way. Our strongly held view counteracts this. This sort of treatment will possibly suffice for a plump, water-filled barrel cactus (although the barrel will not produce new roots in bone-dry soil, not in Arizona desert conditions), but if followed with a much less succulent Joshua tree or ocotillo, it may very well result in the plant's demise. Your new plant needs water, since it is under a great deal of stress resulting from the transplanting process! You do not want to overwater, this is very true. But a deep, thorough watering once a week or so, especially during the long, hot summers, is what will keep your plant alive until it summons the strength to recover from what was certainly a very traumatic move for it. We repeat, water your plants, especially if grown outdoors in a hot desert climate. This advice applies for certainly the first year, and probably for two to three years, depending upon how successfully your plant is recovering and how healthy it was to begin with.
We will caveat these views in just a moment, however, since the watering issue is more complex than it seems. But first, we want to contradict those who tell unsuspecting homeowners that putting a plant into bone-dry soil and letting it bake in hot sun for many summer months with only a "spraying down of the top" is adequate treatment for these plants. If, in fact, the plants do survive, we consider that incidental and due to other factors, such as that it managed to rain in time for the plant to recover, and not that the "spraying down" method was actually ok on its own merits. (By the way, spraying the top is very helpful, as long as the root zone is also watered.) We ask, why does a desert plant, or any plant for that matter, have roots, if it is not to absorb water? Why do ocotillos in nature leaf out within three days of a sufficient rainfall if they did not absorb it through those roots that we so graciously "trim" off, as if the plant did not really need them? It is our contention that the plant-but-don't-water-it school of thought is responsible for more unnecessary and avoidable plant death than any other factor, including the overwatering that this advice is probably intended to thwart. Even a fragment of logic applied to this advice causes it to break down, and we certainly welcome its demise since the ostensible purpose of plant-salvaging was to save the plants in the first place. Right? We apologize for this rant (sort of), but more to the point we want to do our share to send this destructive and deadly myth to its own grave, and save Joshuas and ocotillos from theirs.
We have finished, and will step off the soap box now. Of course, now that we have proclaimed our views, we need to qualify our advice to admonish those who may become overeager with the hose to take it easy, so that the second cause of death, overwatering, may be avoided. It is very true that desert plants, especially those with damaged roots, can easily be overwatered. But first we need to define the term overwatering. Overwatering is a chronic condition, not acute. What we mean is that you cannot overwater most plants by watering heavily one time. Usually, any excess water will drain away, evaporate, or be used by the plant. Overwatering is caused by too much water, too often. Therefore, it is entirely possible to overwater a Joshua tree by giving it five gallons every day for weeks on end, whereas watering it 100 gallons once every month probably won't result in overwatering, even though the aggregate amount of water is similar. Therefore, it is the timing of the water on desert plants that is the critical factor.
A WATERING PRIMER
To wrap up on watering, there is a happy medium to aim for. That is what we have termed the Root Not Rot spectrum. You want to keep the soil a bit moist to promote new root development, but not so sopping wet that you encourage rotting. Too wet, it will decay away, and too dry, the plant sits there, injured and in distress, waiting for it to rain or for you to help it out so that it can regrow the roots it needs so that it can absorb more water to regain its vigor so that it can put out more roots and sooner or later new leaves and ... well, you get the idea. How much to water is simply impossible to say, since it depends upon your soil, your weather lately, the time of year, when you last watered, how big the plant is, how deeply it is set in the hole, and how healthy it was to begin with. We will make a handy reference list to assist you in your quest for success, but you will still need to learn how to read the plant and your own particular combination of conditions.
As a general rule, larger plants need less frequent but deeper waterings than young and small plants. They have a larger mass that stores more water in the tissues of the plant than smaller ones, which is an asset you should capitalize upon. But they need larger planting holes, and the base of the plant is going to be deeper than a smaller one, so the soil in lower layers will not dry out as fast as surface layers exposed to sun and dry winds will. Being deeper holes, more water will be needed at each watering to soak that far down, but it will need doing less often, possibly only once a month in summer, less still in winter.
Sandy soils need more watering than medium-textured soils, and sticky clay soil needs the least of all. Conversely, sandy soils that drain well don't need as much at any one time, although it is hard to overwater in sandy desert soils since excess water simply drains away. Beware of clay soils, however. A large tree in heavy clay might need some water only once in six weeks, if it isn't very hot (defined as regularly 100 degrees or more) where you live, once a month if it is. A small tree in clay soil might need it once a week. A small tree in sandy soil should probably get water every few days in summer, truly so. (We've inadvertently killed a number of small trees in the sandy soil sections of D:FR by inadequate watering, so we speak from experience.) And so on.
Water less in winter coolness than in summer heat. Spring and fall are in between. Once a week in winter, especially if it is near freezing at night, is definitely too often, especially if it has already been raining, as it will in most of Arizona. Remember that you adjust your watering levels for soil, recent weather patterns, and seasonality. Also think about if your plant is exposed to full sun all day long or if it gets some shade at some time. Furthermore, consider the slope. North and east slopes are cooler than south and west slopes or flat areas. This is why we cannot tell you a one-size-fits-all prescription for watering.
Has it rained lately? Remember, the amount of rain that wets the pavement and spots your newly washed car is not going to help a freshly planted ocotillo, not unless it was enough to truly soak in. A good rule of thumb is that an inch of rain will soak average, loamy soil to about a foot deep, if it falls slowly enough to do so and not in a cloudburst of a thunderstorm where much of it runs off. What looks like a tremendous rain to you may not be helpful to a struggling Joshua tree with most of its roots a foot or more below the soil surface. Pay attention, and think like a plant, is the moral of the story. And remember that you do have some latitude here. A little bit too much water or not quite enough on occasion will very likely be compensated for by the plant. But no injured plant can compensate for no water for eight months while the temperature crests 100 degrees for six of them. Nor can they tolerate swampy muck for weeks on end. Balance is the key, and don't forget common sense.
How healthy was the tree to begin with? Be suspicious of severely discounted trees at nurseries: They may very well be aging and the nursery is trying to unload them before they die, thereby putting the burden onto the homeowner. Just so you know, many nurseries will not guarantee Joshuas, ocotillos, or any bare-root plant given the significant failure rate. (A failure rate, we pointedly note, that is made worse by bad watering advice.) Be ready to have a Joshua tree just sit there for several years before resuming growth. The good news is that after one to two years of sitting there being green, it does mean that the tree is alive, since most Joshuas die in under a year. A properly dug ocotillo, i.e., one with a decent root system, will often show some signs of leafing out within three months, although some take two years.
And after all this discussion and effort, your new plant still dies, well, them's the breaks, as they say. It is an unfortunate truth that not every tree or cactus makes it after transplanting. After all, they were really not designed to move like we force them to. They'd really be better off left alone, but since humans are moving into the desert like they are and taking over, we end up moving them anyway, and some won't be able to make it. If a Joshua tree is going to die, most of them will do so within one year or so. But all things considered, with proper watering and transplanting techniques, the survival rate on ocotillos planted with decent roots is about 90% or more, and the Joshua tree re-establishment rate is about 70% to 80%, possibly higher if the trees are moved and planted the same day or in less than five days. Not too bad, considering that the alternative of waiting for 50 to 100 years for the plants to attain landscape size on desert rainfall alone.