Feeding a Trichocereus: what to give and how much
In the ground, usually nothing. In a pot, an ordinary balanced feed at a quarter to half strength about once a month while it is growing, and none in winter.
30-second version first, the whole guide below9 min read
The short version
- If your plant is in the ground, it probably needs nothing.
- If it is in a pot, give it an ordinary balanced feed at a quarter to half the strength on the label, about once a month while it is growing, and none through winter.
- Light comes first: pale, stretched new growth is a sign of too little light, not too little food.
- Soak until water runs out of the bottom each time you feed, so the salts in the feed do not build up in the pot.
- The ratio on the label is the part the advice does not agree on.
Your setup
Pick yours to see what changes in this guide.
Where it grows
In the ground
A cactus planted in the ground generally does not need feeding.
In a pot
Soak until water runs out of the bottom each time you feed, so the salts in the feed do not build up in the pot.
Where it lives
Outdoors
A potted cactus is best kept outdoors where that is possible, because enough light is hard to give indoors.
Indoors
A house interior is usually too dark for good growth without a strong grow light, and the fix for weak, stretched growth is more light, not more feed.
The season
Winter
Feed nothing, and water only enough to keep the plant from shriveling.
Not winter
From spring through early fall, while there is new growth.
Lines are from this guide, or from the cultivar library where one says so. For where they come from, see the end of the guide.
Introduction
If your plant is in the ground, it probably needs nothing. If it is in a pot, give it an ordinary balanced feed at a quarter to half the strength on the label, about once a month while it is growing, and none through winter. Light comes first: pale, stretched new growth is a sign of too little light, not too little food. Below is how to do it, what changes with where yours grows, what the one published experiment on these plants found, and what is known about the extras sold for the job.
How to feed one in a pot
Step 1: Feed only while it is growing
From spring through early fall, while there is new growth. The advice runs from March to October at the longest to June to September at the shortest, so go by your own season: start when growth starts and stop when it stops.
Step 2: Use an ordinary balanced feed
A water-soluble houseplant feed with all the main nutrients is what most of the gardens describe. A feed sold for cacti is another option, and a high-potassium tomato feed is a third. The ratio on the label is the part the advice does not agree on.
Step 3: Cut it to a quarter or a half
Most of the advice dilutes the label strength to a quarter or a half. A quarter strength once a month is one recipe, and half strength once a month, or twice a month, are others. The Arizona-Sonora Desert Museum gives the full label rate once a month.
Step 4: About once a month
Monthly is where most of the advice lands. At the sparing end, University of Minnesota Extension feeds a cactus only once or twice a year. At the generous end, the Arizona-Sonora Desert Museum feeds half strength twice a month.
Step 5: Water it through
Soak until water runs out of the bottom each time you feed, so the salts in the feed do not build up in the pot.
Step 6: Stop in winter
Feed nothing, and water only enough to keep the plant from shriveling. A plant that is not growing takes up little, because roots take up minerals as they grow.
Where yours grows changes the answer
In the ground. A cactus planted in the ground generally does not need feeding.
That advice comes from Arizona, where the soil is desert soil. In dug-over, amended ground it is a strong hint and not a rule: the further a plant is from a pot, the less feeding matters.
In a pot outdoors. The routine above. A potted cactus is best kept outdoors where that is possible, because enough light is hard to give indoors.
Indoors. Light comes before food. A house interior is usually too dark for good growth without a strong grow light, and the fix for weak, stretched growth is more light, not more feed.
Interactive
Light first, indoors
This opens on a plant under a proper light panel. Drop the light dial to deep shade and see what the plant does: thin, pale new growth is what too little light looks like, and more feed does not fix it.
Season
Try a real place
Indoors the light has to do the work of the sun, so the Light dial reads full sun for a proper panel at 750 PPFD or more. A window and a cheap lamp belong nearer dappled shade.
Likely outcome
Fed and watered, it takes the sun
Most of these tolerate all-day sun outside the most UV-intense places, and a plant that is drinking and feeding well stays greener in it than a dry one. Growers who water and feed heavily shade their plants less, not more.
Fed and watered, it takes the sun. Most of these tolerate all-day sun outside the most UV-intense places, and a plant that is drinking and feeding well stays greener in it than a dry one. Growers who water and feed heavily shade their plants less, not more.
Light, roots and salt come before food
Pale, spindly new growth is read as too little light, and roots that have stopped growing take up little. Both are worth checking before the feed.
Step 1: Light, first
Tall, lanky, pale new growth means the light is too low, and the fix is more light. A house interior is usually too dark for good growth without a strong lamp.
Step 2: Then the roots
Roots take up minerals as they grow. In four cacti measured, three to four weeks of dry mix cut the roots' ability to move water to about half and shrank them away from the soil, leaving air gaps. A dry, dormant root takes up little, whatever is in the pot.
Step 3: Then salt
Feed is salts. Growth of cactus pear is halted under saline conditions, and salt reduced the growth of the in the experiment below. That is one reason to water through and to stop in winter.
So sort out the light and the roots first, then feed.
What the one published experiment found
One published experiment looked at plants like these. In 1983 Park Nobel grew seedlings of saguaro, a barrel cactus and chilensis, a relative of the plants sold here, in nutrient solution and varied what they were given.
They responded to nitrogen. Growth reached about 90% of its maximum at a quarter of the nitrogen in a standard full-strength solution, so most of the response came well below full strength.
Changing the phosphorus had a relatively small effect, which sits awkwardly beside the advice some give to feed cacti extra phosphorus.
Salt reduced their growth, at about 100 mM sodium chloride in the solution.
It is one study, on seedlings, in nutrient solution, so it does not settle how to feed a grown plant in a pot. A quarter of the nitrogen in that solution is also not the same as a quarter of the strength on a bottle.
Where the advice splits
No single ratio is agreed. Gardens and extension services recommend four different things: a feed higher in phosphorus than nitrogen, a low-nitrogen feed such as 5-10-5, a high-potassium tomato feed, and a plain balanced feed.
They also differ on how often, from once or twice a year to twice a month at half strength.
A 2019 review of cactus pear grown as a crop says that growing it worldwide, in many soils and for many products, has made the best fertilizer application hard to pin down. It is said of a crop, and it fits the advice above.
The extras sold for this job
Extras sold for feeding fall into three groups that mean different things: tested and failed, tested on other plants with little to show, and not shown on a cactus either way.
Tested on other plants, and it did not hold up
Vitamin B1, or thiamine, is sold to stop transplant shock and start roots. In early lab work it helped roots grown on their own in a flask. On whole plants it has not held up: tests in the 1940s on cuttings, citrus, roses and chrysanthemums found no benefit, and University of California trials in 1983 and 1984 on vegetable transplants found no difference from untreated plants, even with iron, manganese and zinc added. Where a thiamine product with fertilizer in it did better than nothing, the University of California authors put the gain down to the fertilizer, not the vitamin. Healthy plants make their own thiamine. None of these tests used a cactus.
Not shown on a cactus either way
Silicon, chitosan, mycorrhizal fungi, humic acids and fulvic acids are sold as feeding aids. None of the care guides and papers on feeding cacti and succulents tests any of them on a cactus, so they are neither shown to work on one nor shown not to. Mycorrhizal fungi are at least known to live in the roots of a number of cacti.
Tested on other plants, and it did not hold up
Vitamin B1, or thiamine, is sold to stop transplant shock and start roots. In early lab work it helped roots grown on their own in a flask. On whole plants it has not held up: tests in the 1940s on cuttings, citrus, roses and chrysanthemums found no benefit, and University of California trials in 1983 and 1984 on vegetable transplants found no difference from untreated plants, even with iron, manganese and zinc added. Where a thiamine product with fertilizer in it did better than nothing, the University of California authors put the gain down to the fertilizer, not the vitamin. Healthy plants make their own thiamine. None of these tests used a cactus.
Not shown on a cactus either way
Silicon, chitosan, mycorrhizal fungi, humic acids and fulvic acids are sold as feeding aids. None of the care guides and papers on feeding cacti and succulents tests any of them on a cactus, so they are neither shown to work on one nor shown not to. Mycorrhizal fungi are at least known to live in the roots of a number of cacti.
Seaweed and kelp extracts are the middle group: tested on other plants, with little to show. Washington State University's extension review says they can stimulate roots in and transplants in lab and greenhouse work, but that they have no reliable effect on production or on resistance to disease and stress, especially in the field. That review covers landscape plants, not cacti.
What a shortage looks like on a column
Descriptions of a nutrient shortage are written for leaves: yellowing between the veins, older leaves first, edges browning. A Trichocereus has no leaves and no veins, only one continuous stem with the newest tissue at the top.
Those signs do not carry over to a column. If a page tells you confidently what a nitrogen shortage looks like on one, ask where that comes from, and check the light first.
Try it, common mistakes, quick check and sources
Quick check
Question 1: Your plant is in the ground and has not been fed in two years. What do the gardens and extension services say?
Question 2: New growth has come in pale, thin and stretched. What is the most likely cause?
Question 3: A product is sold as vitamin B1 for transplant shock. What have tests on whole plants found?
