What do plants exchange heat with in a vacuum?

Got some pips to plant some bonbon trees in space for me and they grew for a while but at some point heated up to over -15c.

I'm assuming it's because they slowly exchange heat with the tile they're planted in? And is there anything else that could cause them to heat up? Like debris on the floor, liquid on the tile they're planted or critters/dupes walking past them?

I'm using the buildable natural tile mod so am I right in thinking that if I just cool some material to less than -15c and then use that to build my natural tiles, the plants should never go above the -15? Would I also need to cool the seeds down before planting them?

Thanks in advance!