Mammillaria - Shrivelling AND yellowing?

[ShannyK]

Hi everyone,
I've had this cactus for a bit over a year, it's been very happy for most of it but the past month or so I noticed the base has started turning slightly yellowish and soft but shrivelled, and the lower part of the cactus is also shrivelling and slightly more pale. I believe the shrivelling began before the colour change which is what confused me, otherwise I would have assumed it was overwatered. Recently I also noticed there are a few very small, hard orange dots on the lower section where it's turning yellow, not sure what it is? Maybe some kind of infection? The roots have started poking out the bottom, so what I'm thinking now is that maybe because it's outgrown the pot it's drying out and unable to get enough water, but at the same time by trying to overcompensate and increasing how often I water it, perhaps the roots that are in the soil are getting too water logged?

Occasionally it's gotten shrivelled a bit in the past but usually putting more water clears it up, although I've noticed in the past several months it's been looking slightly deflated and adding more water didn't do much, it just made it start yellowing and looking sicker instead. That said, it hasn't stopped flowering and there are still small buds poking up ready go so it can't be that upset..? It's been flowering non-stop and growing steadily for almost a year now! It used to be very plump and bright green though.

I'm thinking of getting a narrower but deeper round pot that I can put a layer of gravel at the bottom of to help with drainage. I'm thinking that if I take it out of the soil, inspect the roots, cut off any that look squishy or rotting, let it dry out and then replant it in dry soil and a bigger pot (leave it for a while before watering) it should recover?

Do I need to wait for the soil to dry out completely first though? I've read that for Mammillaria you're supposed to only repot it when the soil is dry. Or, other sources say you can take it out and let it dry outside the pot before repotting it?

Any help would be appreciated!! This is my first cactus and I'm quite attached to it... I believe this is a Mammillaria polythele.

Climate info - I'm in Northern Australia, so Monsoonal Wet-Dry tropics. Typical temperatures are 29-37oC (85-99 F). We're coming out of our Dry season (very low humidity - basically "arid") into our Monsoon Wet season, and humidity has gone up to 80-90%. It gets strong sun for a few hours every morning then indirect sun the rest of the day. I have to turn it every week or so otherwise it starts growing sideways in the direction of the sun. Every other location doesn't seem to be bright enough though, or it's too harsh and it gets burnt. I don't think it's having a problem with the increasing humidity heading into the Monsoon, it's been through this season before without problems. In fact it started flowering in the middle of the last Monsoon! Although it could be preventing the soil from drying out properly.

[greenknight]

Repotting is a good idea, even before I read what you said I thought "looks like it could use a little more space" - and it's a good idea to inspect the roots if anything seems wrong. I think your theory about why it looks unhappy makes sense - roots poking out the drain hole is a sure sign it need more root run.

No, you don't have to wait for the soil to dry. In general, it's best to unpot any plant when the soil is damp but not too wet, you'll do the least root damage then.

Mammillaria polythele f. inermis, I'd say.

[brixtertabun]

Deleted

[ShannyK]

Ok, thanks guys! I'll repot it this weekend and see how it goes.

I've got another related question though that I've been wondering about for a while - should I be worried about the fact that it keeps flowering non-stop? Flowering is supposed to be seasonal, but this cactus doesn't seem to care. Perhaps because it's always warm/hot here. Just wondering if I'm supposed to try and force dormancy for a few months here and there to give it a break? In case it gets overstressed?

[greenknight]

Mammillaria polythele has a long blooming season, so that's normal. I imagine an annual rest period would do it good, doesn't need to be that long - a couple months without water, maybe. I have no experience growing in that kind of climate.

http://www.llifle.com/Encyclopedia/CACT ... f._inermis

[ShannyK]

Hmm well it doesn't seem to have much ill effect from it right now, although the dead flower buds all over its body look a bit strange. I'll see how it goes after I repot it and fix the yellowing problem.

It may decide by itself to cut back on the flowers and focus on putting out new growth once in the new pot, although it was growing really quickly even while putting out flowers earlier this year. Maybe next Dry season I'll be a bit harsher on it and cut back on the water, see what happens.

Thank you

[ShannyK]

It accidentally got rained on today, didn't catch it in time. So I'll try your trick of digging it out and letting the roots air dry for a few days before I put it in a new pot.

[DaveW]

An old gardeners saying for potted plants that were not looking as they should was "when in doubt knock it out", meaning knock it out of the pot and examine the roots. That way you see if they have root mealy, if the roots are dying, or the base starting to rot so can catch it in time to do something about it, as the others have already said. However the plant in your picture look OK to me?

Cacti are not like leafy plants and can stand around dry root for a week or more before repotting. Also you can repot them if required at any time of the year, including their normal dormant period provided you use dry soil and do not water again until the start of the growing season. As you say, if a plant pot gets soaked at the wrong time of the year you can unpot it, let it dry bare root for a few days then repot in dry soil.

[greenknight]

I agree, it looks pretty healthy. Cacti grow only at the tip(s), the old part at the bottom tends to get faded eventually - except on globular species, on those, the oldest part shrivels up and disappears under the plant, this is how they stay short. On columnar species, the base eventually becomes corky and brown, though this species doesn't seem to be quick to do that.

However, that this one has changed noticeably over a fairly short time span is, I think, a sign the cactus is having a hard time getting enough water and nutrients, so it's robbing the older tissue to support the growing, vigorous part. Probably just from being under-potted, but you should inspect the roots for any problems, like DaveW said.

The new pot should not be too much bigger, these are shallow-rooted plants - maybe a couple cm. larger than the existing root ball. Figure on repotting again in about 3 years.

[ShannyK]

Well I pulled it out today and it was DEFINITELY extremely rootbound, poor thing. Roots everywhere there was possibly any space at all. Managed to salvage the bulk of it although there were lots of casualties.

Ended up putting it in a much bigger pot than I anticipated because there were more roots than I expected! It's almost three times the size of the old pot so I hope it's not too big. But I figured if it keeps growing as vigorously as it did over the past year once it recovers it's likely to outgrow a smaller pot within a year again.

Good news is the roots all looked very healthy to me, I don't think there was any evidence of mealy bugs or anything else nasty. So hopefully now it's not so cramped it will recover and go back to normal.

Thanks everyone for your tips! It was so helpful.

[greenknight]

Good healthy roots, I agree. Reasonable size pot, should do fine. Wait a couple weeks before watering to give the roots a chance to heal, that reduces the risk of rot. It won't ned water as often now, until it gets that pot filled with roots.

[ShannyK]

Will do. Thank you again