Originally published in the San Diego Cactus and Succulent Society Newsletter for October, 2021.
We always remember our first love – and for me, that’s the blue Pilosocereus pachycladus.
You may have seen them listed as Pilosocerus azureus, but as far as I’m aware, that’s not a currently valid Latin name. For this article, I’ll be specifically talking about P. pachycladus, but the care can be applied for P. glaucescens, P. magnificus, and/or nearly any other of the blue columnar species from Brazil.
Pilosocereus pachycladus hails from Brazil, in the roughly north-eastern interior region where there is caatinga.
The caatinga is a dry, desert shrubland and a very specific biome found only in Brazil. The region typically experiences a very hot and dry winter, with a cold and rainy summer. If that weather pattern sounds familiar, it should! It’s incredibly similar to our growing conditions here in San Diego – just swap the names of the seasons, and we’re set.
In Brazil, these cacti would rarely experience temperatures below 50°F, and during the peak of the dry and hot season, ground temperatures can be up to 140°F! Water arrives sporadically during the wet season and can average as little as 5” a year to over 30”.
With a species so well adapted to high temperatures, extreme moisture fluctuations, and tolerant of cold rain, is it any surprise these are excellent candidates for many San Diego xeriscape gardens?
To thrive in our region, the Pilosocereus needs careful consideration as to where they can be planted and still thrive. While in their native region, they rarely (if ever) experience temperatures below 50°F, that is a near-nightly occurrence here in winter.
Fortunately for us, these cacti are much more tolerant of cold weather than you might suspect. I have been growing a handful of species for a few years, and they have all tolerated brief dips into the high 20s or low 30s for short periods of time.
For best appearance, if it does dip below 35°F, you should plan on covering your cactus with frost cloth, or at least covering the tips of the arms. I’ve seen plant friends use Styrofoam cups to protect their cactus arm tips, but I haven’t had much luck with that method (and the grocery store cups are never big enough!). These cacti are particularly prone to mild “frostbite”, and the growth points will scar up or die back a bit.
If you want a heavily branched specimen, this might not be a bad thing: the damage to apical growth points will encourage additional arms to come out!
My oldest specimen had only two ‘branches’ for years, up until it experienced damage at the apical growth points when I finally planted it in the ground.
Pay close attention to the conditions on your property or around your home when deciding where to plant your Pilosocereus, if you’re placing it in the ground. Higher up on a hill, where water will naturally drain away from the roots and it is less likely to experience frost is a good option. If you’re more inland and experience temperatures below 32F more than a handful of days a year, keeping your Pilosocereus closer to your house can make more sense. Sheltered by a wall or close to your walls on a south-facing slope can be an ideal placement to shelter your beautiful blue cactus through the coldest months.
In summer, it’s hard to say that there’s a day too hot for these! They’ll not just tolerate hot days of 100°F or more, they’ll love them.
For best appearance and most robust growth, water them regularly during the hottest months, and you’ll be rewarded with a large, beautiful cactus and profuse blooms.
In pots or in the ground?
Much of my growing advice may sound like it’s for those who want to plant their P. pachycladus in the ground, but it works if they’re in pots, too!
The most challenging part about growing these cacti in pots is that they really, really want to be cactus “trees”. In habitat, they can reach heights of over 10 meters tall – over 32 feet! When growing them in pots, we’re treading close to how bonsai gardeners carefully maintain a healthy (but much diminished in size) tree.
Many growers keep Pilosocereus in pots very successfully for many years, although potted specimens will never reach the same heights that in-ground specimens will. Growth will be significantly slower once they’ve maximized their available root space, and they can be much more sensitive to poor watering routines.
My largest and oldest specimen shot up by multiple feet the first year I potted it up from a 1 gallon to a roughly 3-gallon pot! After that initial spurt, however, growth slowed significantly, to barely a foot the following year, and mere few inches the year after that. Fertilization with worm emulsion and cactus-specific fertilizer had little effect, and it would dry out within a day when watered even if it wasn’t that hot out (a classic sign of a root-bound or under-potted plant).
In bonsai cultivation, that would be the time to repot it and refresh the soil…but how feasible is that when we’re talking about a cactus taller than you are?
For cactus culture, you’ll have to weigh that eventuality for yourself. In a decorative pot, with additional soil added on top each year along with a cautious fertilization routine, you could have a beautiful cactus for many years. It may not get terribly tall, but if you’re careful, it’ll be beautiful for quite a long time!
If you, like me, want to see the cactus reach its maximum size and growth potential, stick it in the ground! I highly recommend doing so before it’s reached the taller-than-you-are stage, as it’s much easier to maneuver and you’re far less likely to damage the cactus in the transplant process.
Should your in-ground cactus get too large, branchy, or unwieldy, you can use a hedge trimming saw to cut off arms to give away. Trim them in spring, when it’s warming up and the cactus is entering its growth season. Let the cut edges callous over for a few days or a week, then stick them in a pot just barely bigger around than they are and fill with 50/50 cactus soil and pumice. They should root within a month (or three), and you can give them away to fellow club members!
Additional Growing Tips
There are a few things you can do to see the best success with your Pilosocereus, especially if you want a large, sculptural specimen.
Just because they grow in rocky, sandy soil in habitat and have a very brief wet season doesn’t mean they must be grown the same way in cultivation. In fact, for the best looking Pilos (especially in-ground), water them regularly all through summer! My very rough rule of thumb is if the average week is over 80°F, I water them once a week. If it’s over 90°F, make that maybe twice a week. If other plants near them need water, they get extra.
If we’re experiencing a heat wave of 100°F or more, I make sure to water a day or two before the temperatures reach 100. When we have heat waves with multiple 100+ days, I’ll water as often as every other day or every third day! During periods of such high heat and intense sunlight, the cacti seem to turn all that moisture straight into growth.
For our growers in apartments or with limited yards, my advice on watering changes dramatically based on the amount of sun your cactus gets. If it’s not experiencing full sun all day, you’ll need much less water than I’m suggesting here – especially if it’s on a partially shaded patio or is shaded by building shadows for part of the day.
I also want to emphasize not waiting until your cactus is taller than you and, most likely, severely pot bound before planting it in the ground (if that’s your goal)! The shock and time needed to recuperate from the abrupt change in conditions is pretty rough on your cactus. It took my 6-foot tall Pilosocereus nearly a year to finally start showing new growth after being planted in-ground.
Fertilization is entirely unnecessary in-ground, but the cacti do seem to appreciate it when potted. I saw the best results using fish emulsion at half strength, but if you’re not a fan of the smell, a dilute balanced fertilizer would probably work just as well.
Try to avoid touching the body of your cactus if you can. They develop a glaucous coating with their new growth, but only when they are newly growing. Once you’ve touched the coating and wiped it off with a finger, it’s gone for good.
You can eat the fruits they make, but they’re not particularly tasty. If you’re looking for a glaucous blue cactus that gets massive and produces tasty fruits, I highly recommend the Peruvian Apple Cactus.
I hope this encourages our members to seek out and grow some blue beauties of their own! They’re a rewarding species that are easier than they may seem.