The green leaves look good. Old leaves will yellow and eventually dry up and fall off. Was it dry? Or soaking wet? It may need a haircut - I trim mine once a year to keep them smaller for the winter.

It was fairly hot and dry this summer here, but I kept it watered. I know arrowheads like a fairly moist soil, but not soaking.

I have a pan of them, a few pots, and then a drinking cup to keep them separate. Kind of weedy but I like that.

I used to keep big pots for them but that became too much.

I would keep on doing what you are doing. Maybe add some fresh potting mix on top with a little bit of fertilizer (slow release or water soluble).

The first suspect would be the contrast in light levels between where it was sited outdoors and what new light levels are indoors. A plant's ability to adjust to changes in light levels is not unlimited. Something I wrote about a plant's ability to adjust to changes in photo loads:

Light Acclimatization

The degree to which a leaf can 'acclimate' to changes in photo load is limited. IOW, you can't expect a leaf that emerged under a full sun light load to acclimate to light levels in a dim corner, any more than you can expect a leaf that emerged in a dim corner to acclimate to a full sun site; this, no matter how long you allow for gradual acclimation. Too, the plant's ability to adapt to higher light loads is greater than its ability to adapt to lesser light loads.

Using a 1-10 numerical example to illustrate (numerical value of 1 is low light, 10 is high light): If a leaf emerges where the light level is measured at 5 units, it's range of adjustment might only be to 3.5 on the low side, but 8 to 8.5 on the high side. When the level of adjustment required is greater than what the plant is genetically programmed to deal with, the leaf is shed, and this can occur with conspicuous or relatively inconspicuous damage to the leaf. While shed leaves can never be replaced, per se, new branches can form (with new leaves along the new branches) in the axils of dying leaves or from latent buds immediately above scars left by shed leaves. Too, whenever a new leaf appears it will be perfectly acclimated to whatever the photo load is where the plant is positioned (within the limits of what it is genetically programmed to tolerate).

The regulating mechanism that determines whether leaves are able to adapt or destined to be shed centers on the flow of a growth regulator, auxin, across what is called the abscission done, located at the base of the leaf petiole (stem) where the leaf is attached to a branch. Steep declines in the rate of auxin flow causes an abscission layer to form, which effectively forms a wall between the dead/ dying leaf and viable plant parts, soon after which the leaf separates from the rest of the plant.

Al

Thanks for all the comments.

Yesterday, I took some photos of the syngoniums living in an aluminum tray for the last few years when I was outside bringing plants in for winter. I also have golden philodendron in the same tray, but to me, I think it is golden pothos.

If I lived in Florida, they would probably be in the ground and taking over some of the trees like English Ivy.

i suspect it got colder than you thought outside.. before you brought it in ..

house temp is fine ...

moving anything in or out of the house is a shock ... a stress .. but you wouldnt expect that much leaf loss ..

do you recall.. was it al the tallest top-,ost leaves... those which would have got hit with cold night air???

as its stressed.. fert is not really a response at this time ...

and.. if this all happened in the last week.. its probably not a light issue.. that takes months of low light to appear .. you dont lose two dozen leaves in a week ... soooo im back to a cold event outside ...

and it will get over it..l as long as you dont love it to death.. wate3r properly and leave it alone ... and if in jan you see it tilting toward the light.. then you have low light ..

ken

The most leaf loss was in the mid-center of the plant and along the tendrils.