I’m super excited to be writing about this particular species because it is my absolute favorite tree. I fell in love with desert ironwood (Olneya tesota) at the very beginning of my landscaping career. I was managing a large property that had just one on site, a big specimen located in a wide expanse of lawn. It was off-season, so at first it was just one of what seemed like thousands of trees I had to study and learn.
The Sonoran Desert, home of the ironwood tree, at night. Foreground: Skeleton of a self-preserved ironwood tree.
The first time I saw it in-bloom, I didn’t recognize it. I saw it through the windshield of my truck as I was parking, and for a second I thought I was in the wrong place. As I walked toward it, I figured out it had bloomed, and I was excited. It looked so beautiful from my vantage point, and the closer I walked, the prettier it became.
The first time I saw an ironwood tree bloom, I was mesmerized. The gauzy lavender cloud of flowers appears to bathe everything around it in a soft purple glow. “Olneya tesota” by Matt Lavin is licensed under CC BY-SA 2.0
The entire canopy was hidden underneath what looked like a shimmery cloud of this incredible shade of pale purple with a hint of dark pink. Instead of being on the limbs, the blooms looked almost like they were hovering in the air around them. There was movement. There was dimension. It was the most gorgeous thing I’d ever seen, and I fell in love.
The more I learned about this enigmatic tree, the more I loved it. So I’m excited to share the desert ironwood with you because, being endemic to the desert southwest, there’s a good chance you don’t know about it. And to truly appreciate this tree, we must understand not only its individual history, but also the history of its habitat: the Sonoran Desert.
How the Sonoran Desert Became Home to the Ironwood Tree
This map shows all the deserts in the US. There are four main, but they have been broken out into smaller parts.
1 – 8. Great Basin Desert 9. Mojave Desert. 10 and 11. Sonoran Desert — where ironwood trees occur naturally. 12. Chihuahuan Desert
“Desert ecoregions of North America as defined by the Commission for Environmental Cooperation.” By Joe Roe / This file is licensed under the Creative Commons Attribution-Share Alike 4.0 International license.
The Sonoran Desert (one of only four North American deserts, all of which are in the far western third of the US) likely began forming in the Miocene Epoch of the Neogene Period. Approximately 23- to 5-million years ago, the already-formed Sierra Madres and Rocky Mountains were pushed high enough to disturb atmospheric flow. For the first time, tropical moisture from both the Gulf of Mexico and the Pacific Ocean was impeded by the two mountain ranges, thus drying out the area of land between them.
Also key to the Sonoran’s present-day topography is the state of Baja California in Mexico. This piece of land used to be part of Mexico’s mainland but was dislodged due to seismic activity along the San Andreas fault. After it broke off, it floated over and eventually attached itself to the bottom of California, forming the Baja Peninsula and the Gulf of California.
The ironwood tree was first described by botanist Asa Gray in his 1854 collection Plantae Novae Thurberianae. He named it Olneya tesota after his friend and fellow botanist Stephen Thayer Olney.
Shortly after the beginning of the current interglacial (the Holocene), about 11,000 years ago, plant life like brittlebush and saguaro cacti (Sonoran Desert mainstays) began showing up in the area where the Sonoran’s development had been stymied by more Ice Ages, glaciers, then the subsequent melting of those glaciers. Despite the early return of these desert plants, the area remained heavily populated with woodland plants for another 2,000 years. Those plants eventually retreated upslope, and the Sonoran Desert’s latest iteration, the one we still know today, was complete.
It was thought that Olneya tesota formed concurrently with the Sonoran beginning in the mid-Miocene Epoch but some paleogeological findings indicate that may not have happened until the mid- to late-Holocene Epoch (the current epoch).
The Significance of the Ironwood’s Role in the Sonoran Desert
A relatively young and small ironwood tree as it occurs in nature. The “ironwood” common name is a reference to its incredibly dense wood. Because of its density, it is very hard, heavy, and does not float.
“Not sea worthy wood!” by Take A Hike Arizona is licensed under CC BY-NC-ND 2.0
The Sonoran Desert is teeming with life, rich in diversity, the most diverse of any North American desert. At least 60 species of mammal (including the country’s only Jaguar population), 350 species of bird, 100 different reptiles, 20 amphibians, and another 30 species of native fish all make their homes here. Additionally, 2,000 different species of plants, flowers, trees, and cacti all thrive in the Sonoran Desert, which is the most of any desert in the entire world.
In this biodiverse ecosystem, the ironwood has responsibilities and plays an important role in the successful continuation of life:
- As a keystone species: keystone species are those whose presence is needed to hold the entire system together; without them, the system would fail.
- As an indicator species: indicator species are used as a barometer by which certain environmental conditions are measured; they can tell us that something has changed or is going to change in the environment.
The seed pods of this tree have historically been an important source of food for both animal and human alike.
- As a nurse plant: perhaps most impactful of all its responsibilities. Ironwoods are “nurse plants,” meaning they’re important to the survival of other life forms; in this case, about 500 species rely on the ironwood for their continued existence. Ironwoods provide a place of refuge for animals being hunted, they provide shelter, they’re a source of food, and their dense, low canopies create a microclimate around them that’s about 15-degrees cooler. More than 250 different species have been documented starting their growth in the nutrient-rich soil under an ironwood tree.
Before Planting: The Pertinent Details
Before getting too excited, or even going to the nursery, you want to ensure whatever species you’re considering is right for you. Not only that, but that you’re right for it. It’s important that you both be happy, and that you both possess the wherewithal to facilitate that happiness.
Ironwoods are easy to grow and very low-maintenance, but they have a couple of personality quirks that could be deal-breakers. So here’s a quick rundown:
- Ironwoods typically reach a height of anywhere from 25- to 45-feet
- They’re members of the pea family, therefore, they’re leguminous and produce seed pods
- Trunk diameter can get up to 24″ across
- They’re exceptionally slow growers (I cannot emphasize this point enough; they are the slowest-growing tree I’ve ever worked with, by far)
- Ironwoods like to live long lives, sometimes making it to 1,000 or even 1,500 years old
- They’re known for their dense, heavy wood (hence the common name of “ironwood”)
- They have thorns — a pair of them at the base of every leaf — making pruning a bit of a tough job
Without any trimming at all, an ironwood takes on the shape of an oversized, overgrown shrub. In ornamental applications, natural-style pruning is desirable.
- Speaking of pruning, they require a lot of it to look tip-top (more on that later)
- The bloom period is short, usually 10-18 days, sometime between April and June
- They’re evergreen, but will drop its leaves if too cold or too thirsty
- They love very coarse, very porous soil; if your existing soil is neither coarse nor porous, it needs to be amended before planting
- Ironwoods are hardy to 20-degrees fahrenheit (they don’t like temperatures below 20-degrees)
How to Plant and Grow an Ironwood Tree
The flowers of an ironwood tree, shot at close-up range. The tree blooms for 10-18 days once per year between April and June.
Ironwoods can be started from seed, right out of the pods taken from an ironwood tree. But just because they can, doesn’t mean they should.
Buy a nursery specimen
Waiting for seeds will set you back a boatload of years; it’ll take you up to ten years to grow a specimen roughly the same size as one you could pick up at a nursery tomorrow. It could be 15 years or more before a specimen grown from seed flowers or produces fruit.
15-gallon vs. 24″ box (hint: get the 15)
My recommendation is to skip the starting-from-seed on this species and support your local nursery industry by buying from them. Typical install size is 24″-box or 15-gallon-bucket. The 15-gallon size container plants actually adjust faster than bigger specimens, plus, there isn’t usually that much difference in size. In this case, I’d save the money and go with the 15-gallon size.
When: plant in October
Where you plant an ironwood tree is just as important as how you plant it, and both are just as important as when. In this case, October is the best time to plant your new ironwood. The temperatures below ground are still nice and warm from summer but already much cooler above ground, which creates excellent conditions for planting. Plus, October is early enough to give the tree time to settle in a bit before cold winter temperatures.
Where: front and center
Ironwoods don’t lend themselves well to being tucked back into a corner or used as filler to take up space. Their job in nature may be to take care of everyone and everything else, but in an ornamental application, they like the starring role, and they play it well.
If you can stay on top of pruning and are willing to outfit it with some low-voltage landscape lighting, give it a wide-open space where it can shine and be the undisputed statement piece. With careful, detailed, selective pruning, ironwoods are architectural masterpieces. In the low light of fading sun, they create interesting silhouettes, and uplit with soft lighting, they look rich and luxurious.
How: about the same as usual (but mind the roots!)
Prepare a hole that’s roughly the same depth as the root ball, or just a hair shallower (some of the root ball sticking out of the ground is OK), and two or three times as wide. As you dig, turn the soil and break apart any clumps.
The soil should be coarse and porous to allow for maximum water and air circulation throughout the root system. If the soil needs to be amended to accomplish this, now is the time to do it.
From container to ground
Looking up into the canopy of an ironwood tree. When used ornamentally, structural pruning is required to remove cross branches.
There are no secret tricks to getting the tree from the nursery bucket to the hole in the ground. One way uses a straight-edge to slice off the bottom of the container, then cut up the sides and pull it away while the tree is in the hole. Depending on how worried you may or may not be about damaging roots, this method may or may not work for you.
Another method involves turning the container on its side, shimmying the container off the plant, and rolling the plant into the hole. Not always easy to do and keep the root ball together at the same time.
My experience has been that the easiest way is to simply leave the container and tree standing upright, and give the container a few knocks around the outside to loosen the root ball away from the container walls. Then grab low on the tree trunk and give a little tug to make sure it’s loose, and, assuming it is, just go ahead and pull the whole root ball up and out. Ideally, do this right next to your hole, so you can pull up and out of the container and then set it down into the hole in one fluid motion.
Roots: touch them or leave them alone?
Roots that are white in color, not completely dried out, and stay together when touched are still alive and relatively healthy, despite being bound.
There are at least two schools of thought on how to treat the roots when planting: do not disturb them at all no matter what, or slice into them with a straight edge. Neither method should be used absolutely. Something like this should always be species-specific. Educate yourself on the preferences of the species you’re planting so you know which method typically gets the best results. Even armed with that knowledge, do a visual inspection.
- Pull the root ball out of the container, set it on the ground, and give it a thorough inspection. You’re looking for the appearance of healthy roots versus damaged roots.
- If the roots are thick and white, they’re new and healthy.
- If they’re stringier and darker brown in color, and growing around the root ball, use your fingers to lightly disturb them and see how they respond.
- If they fall apart easily with a light touch, that section of the root system is useless. But..!
- *Look for a point above it where there’s new healthy growth that doesn’t come apart. As long as you have a good amount of healthy roots still there, the plant should be fine.
- To be extra-sure, and to give the tree a little boost, water in with SuperThrive after planting.
In the specific case of ironwoods, they have two kinds of roots: a shallow, spreading root system and a deep taproot. The shallow part is outfitted with nodules that deliver nitrogen to the soil, and the deep taproot goes down and finds water. If a visual inspection doesn’t do the trick, touch the roots only lightly to test their structural integrity. As long as they’re not coming apart in your hand at your touch, you should be ok.
Roots that are brown, dry and brittle to the touch, and that disintegrate or fall apart when lightly disturbed are dead and cannot be planted.
Once you’ve checked the roots and have your root ball in the hole, use one hand to hold the tree straight upright and start backfilling the hole with the other hand. Stop every few scoops and tamp down to release any air pockets. When you’re done backfilling, a small lip of root ball should be sticking up just a smidge higher than the ground.
Once the hole is backfilled, there are a few more tasks to complete. Follow this after-planting checklist to make sure you don’t forget anything:
- Remove the nursery stake (if applicable)
- Restake using proper methodology (if needed)
- Water the tree in, giving it a deep soaking
- Put a couple of capfuls of SuperThrive into a full 1-gallon-jug of distilled water, shake to incorporate thoroughly, then pour the entire gallon jug over the planting area
- If adding to an irrigation system, place the drip emitter and test from the clock to make sure it’s working
- Top-dress the planting area with a 2-inch layer of mulch
Ironwood Tree Maintenance
Ironwoods are “high” maintenance the first year they’re establishing (relative to the rest of their lives) and virtually hands-off after that, apart from pruning. Watering and pruning are the only two things you’ll have to consider.
It’s not uncommon for the desiccated trunks of old ironwoods to remain standing for hundreds of years. They contain a toxin that makes the dense wood virtually non-biodegradable.
The first year: dig a berm all away around the tree, about two feet out from the trunk. Water the tree every few days, and fill in the berm once per week.
After a year: the berm should be moved out as the tree grows. Give the tree a deep soak once every two to four weeks. If this doesn’t appear to be enough, you can increase, but no more than once per week. Every time you water, fill in the berm.
Two years and older: continue to move the berm out once a year and fill with each watering. This encourages deeper root growth. Once the tree has doubled in size, the water can be cut back more.
Note: Ironwood trees are incredibly drought-resistant, and do not like too much water. Be attentive and keep an eye out for signs of overwatering.
Once the tree is about three years old, start implementing a pruning schedule. The key to a really good-looking ironwood is in the pruning. Ironwood trees are amazing, but left to grow naturally can look a bit disheveled and messy. The more thoughtful and deliberate you are with your approach, the bigger the payoff will be.
The video below provides excellent instruction on how to properly prune a multi-trunk desert tree.
Multi-trunk vs. single trunk
Ironwoods are naturally multi-trunked, and there’s no reason to change that. The multi-trunk design is perfect for their upward and outward (and then downward) growing pattern. You want to let the canopy be as natural as possible, and because it naturally wants to spread out, a single trunk just isn’t practical or visually pleasing under all that spread.
Go slow, be selective
If you’re going to hire professional tree trimmers to maintain your ironwood, then you don’t really have to worry about this. But it’s something you can learn to do, and you might find that the activity is satisfying and fulfilling in ways you didn’t expect. If you do it yourself, it’s OK to go slow. Take a step back, look at the tree, and notice where your eyes fall first. What it notices first. Is it noticing something nice that should stay or something unsightly that needs to go?
Ironwoods have a naturally pleasing structure. Just lift from the bottom, create some air and space between the limbs, and follow their natural shape. You’ll do just fine.
Adding uplights to trees is a fast and easy way to add instant visual interest to your landscaping. It looks expensive, but it’s not.
Ironwoods don’t have issues with pests or diseases, so the last thing we’ll talk about is lighting. Lighting is, of course, always an optional upgrade, not necessary for the health of the plant. It doesn’t hurt it, and it doesn’t add to it. It’s just a matter of personal aesthetics.
Ironwood trees happen to have a structure that’s perfect for strategically placed uplights. It doesn’t matter what style your overall design is, or if the ironwood is the only thing in your yard, it creates such a stunning effect that’s lovely to look at.
Many low-voltage systems these days are solar-powered or plug-in, manage to stay discreet during the daylight hours, and are reasonably priced.
A dead ironwood tree, perhaps lamenting the loss of its beautiful blooms, and in so doing failing to see the beauty it still possesses.
If you happen to live in the desert southwest and have never worked with an ironwood, I hope you give this slow and steady beauty a shot. Even if you’re not in the southwest but within zones 8-11, this could be a fun experiment.
The bloom cycle may be ephemeral, but it’s an experience millions of years in the making that stays with you forever. And maybe it’s a good reminder that we should always be willing to look deeper, and find the beauty where we may have missed it before.