Ask a Question forum: My cactus has brown spots on it.

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Jan 7, 2018 9:26 AM CST
I have a large collection of cacti that are set up on my windowsill. They get lots of light, from about 7:00 to 1:00. One of my cacti is a Paddle cactus. You are not supposed to water a cactus often, but every week I water it, it guzzles down the water. I have started to notice brown spots on the side facing the window. This may be because of the cold, because it is very cold and has been in the negatives the previous week. I have moved all my cacti farther back from the windowsill just in case. Do you know what may be causing the brown spots on my cactus? Is it odd that the cactus is guzzling down the water every week?
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[Last edited by NdMaPlants - Jan 7, 2018 3:10 PM (+)]
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Name: Daisy I
Reno, Nv (Zone 6b)
Not all who wander are lost
Garden Sages Plant Identifier
Jan 7, 2018 2:57 PM CST

You may have diagnosed your own problem - cold damage., especially if the damage is on the side facing the window. But, it could also be an indicator you're overwatering. In winter, and especially when its cold, once a week watering is way too much. That pot is very large for the size cactus (BTW Opuntia microdasys) so will retain moisture for a long time. One of the absolutes to healthy cactus growing is to allow the roots to dry between waterings. The other is adequate light.
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and proclaiming...."WOW What a Ride!!" -Mark Frost

Jan 7, 2018 3:06 PM CST
Thanks for the reply, @daisyl. I have a smaller pot that can I can easily plant it in. Is it odd that the plant is guzzling down water every week? The water is literally gone right after I water the plant. Should I still bump back its watering schedule?
Name: Daisy I
Reno, Nv (Zone 6b)
Not all who wander are lost
Garden Sages Plant Identifier
Jan 7, 2018 3:49 PM CST
Wait until spring to repot cactus - they recover much faster if they are in active growth.

Its not the plant that's guzzling water. Well, let me re-state that. A cactus will absorb every molecule of water it can get its roots on. They live in the desert where every drop is precious and needed. Most plants come with shutoff valves - when they've gotten their fill of water, they shut off the valves. Cactus don't think they EVER have enough so you have to be the adult. Don't water until the soil is dry 2 or 3 inches down.

Cactus also do better in wider shallower pots as their roots fan out on the surface. Use a soil that drains very quickly. I use cactus soil with added perlite. You don't want any moisture against the roots between waterings.
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and proclaiming...."WOW What a Ride!!" -Mark Frost

Name: Philip Becker
Fresno California (Zone 8a)
Jan 10, 2018 3:14 PM CST
I double down with @Daisyl
Anything i say, could be misrepresented, or wrong.
Name: tarev
San Joaquin County, CA (Zone 9b)
Always count your blessings in life
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Jan 11, 2018 10:10 AM CST
Hello NdMaPlants, it is likely getting overwatered. Typically my personal guide for my succulents and cacti, especially when they are indoors during winter, if temps are colder and light levels are shorter, greater interval in watering.

Weekly watering indoors at this time of the year is not conducive for this plant. It is different if it is cold weather hardened growing outdoors where there is more influence of air and light around it. It also naturally slows down in growing during the winter months so all the more watering should be minimal.

At times, leaving them alone during winter is the hardest thing to do, but got to change and adjust watering regimen as the seasons change.
Name: Will Creed
Professional indoor plant consultan
Jan 11, 2018 2:53 PM CST
I agree with Daisy in all respects, but unless you are experienced with Cacti and repotting, I would not repot it even in the warmer months. If done incorrectly, you could do a lot of damage.

The alternative to a smaller pot is much less frequent watering - no more than once per month even in summer.
Will Creed
Horticultural Help, NYC
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Name: Baja
Baja California (Zone 11b)
Cactus and Succulents Seed Starter Foliage Fan Xeriscape Container Gardener Hummingbirder
Native Plants and Wildflowers Garden Photography Region: Mexico Plant Identifier Forum moderator Plant Database Moderator
Jan 11, 2018 3:37 PM CST
If you don't ever repot the plant, it will end up frustrated, root bound, inhibited, and stalled out. These plants can easily fill a 14 or 16 inch pot, over several years. They are shrubby in the ground. The pot it's in looks okay to me for now, for a year or so. The mix maybe could use a bit more perlite or pumice to improve drainage. Be sure to repot during spring instead of winter, and do not water for a week afterwards. Do it in small steps for the best results. Too big a pot with not enough roots in it, and the soil will stay wet for too long, giving you more risk of rot.

The apparent "guzzling down" of water may have to do with the fact that the top layer of the soil dries out sooner than the soil at depth (which is what matters when you're trying to figure out how often to water). The top will appear dry long before the bottom actually dries out. When in doubt wait to water, especially in winter with low light and cold temps, but you can try using a moisture meter, your finger, or a chopstick to burrow down and see what's happening underneath. Just don't do that every time or you will start to damage the roots.

My main piece of advice with this plant has to do with those tiny spines (technically they are called glochids). They are incredibly dangerous. Should any human or pet brush up against the plant by accident, they will pick up dozens of those little spines and then go through the annoying and tedious process of trying to remove them. So come spring when your plant is better, and maybe you're thinking of giving it some outdoor exposure, be sure to find a location away from traffic. My Opuntia microdasys plants were pretty big by the time they eventually were banned from the patio for bad behavior. Smiling

Flower here:

[Last edited by Baja_Costero - Jan 11, 2018 4:24 PM (+)]
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Sep 6, 2018 12:44 AM CST
I have rust like marks and some leaves have been nibbled at, I can scrape the rust like marks off, it is on a few of my plants

Nov 4, 2018 9:07 AM CST
Hello Mam, please advise how to remove brown spots from my cacti?
Ohio (Zone 5b)
Nov 4, 2018 11:54 AM CST
I just have a future insert here- when repotting cactus with spines, wrapping them in newspaper, and using gloves meant for roses (to protect against thorns) and metal kitchen tongs help greatly!

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