Ruined Home is swapped from Fall to Spring likely because Asgore is now the caretaker of Ruined Home instead of Toriel.note While Toriel did tend to some plants successfully, it wasn't really her "thing": she tends to the bed of golden flowers, and she has a couple potted golden flowers in her home along with a water sausage and a cactus. Cacti are notoriously hardy, making it an ideal plant for someone who might be too consumed by depression or duty to remember to take close care of it. The water sausage seems to be a recent gift from Sans (as it's implied by some of Papyrus' dialogue that Sans tried out a pie recipe she gave him, under Narrator Chara theory Chara does not recognize it from their own time with Toriel, and Sans in turn is associated with water sausages from selling them), so naturally she'd be taking better care of a gift from her Only Friend than she would random plants. Likewise, the golden flowers she keeps as potted plants are a Tragic Keepsake as a symbol of her beloved children and of Chara's Last Request, and the bed of flowers at the start even marked Chara's grave; like Sans' gift, Toriel would have good (sentimental) reason to put effort into tending the golden flowers (and she seemed to visit daily to check for fallen humans anyway, giving her plenty of time to care for the flowers). Outside of her house and the golden flowers, there's little to no plantlife in the Ruins, only several bunches of fallen leaves from what looks like it could be a dead or dying tree in front of Torie's house. This makes sense when you remember how isolated and depressed Toriel was as the queen in self-imposed exile, and angry at other monsters for going along with Asgore's war declaration. Gardening and maintaining morale are much more Asgore's "things." In Undertale he kept a garden before his children's deaths where they played, it's implied Chara took their interest in gardening from him and his example, and in current day the garden is overrun with golden flowers and even his house is full to bursting with potted golden flowers. Where Toriel locked herself away in her grief and only put effort towards certain things, in contrast Asgore seems to push away his grief by focusing all his time and energy into gardening (what he's doing when Frisk shows up) and connecting with the community around him to avoid dealing with his own pain (visiting Monster Kid's school, chatting up monster civilians, and being on friendly terms with everyone outside of Toriel, but not being able to bring himself to ruin everyone's hopes by reversing his war declaration). In TS!Underswap, Ruined Home is a much smaller space than the entire expanse of the Underground his subjects had before; not only would he still be as friendly as Undertale Asgore was with his subjects, potential overcrowding and the mental state of the population of Ruined Home would be a more pressing issue for Asgore in the isolated Ruined Home than for Undertale Asgore (where it's implied he still dealt with similar issues, but at least had much more space to work with). Finding ways to make the old city spacious and functional and making the people's surroundings more beautiful and cheerful with flowers, would be a bigger priority and one he could occupy himself with when humans aren't present.
Starlight Isles (the replacement of Snowdin) is switched from Winter to Summer. While there may not be a character reason for the swap like the above, at least the way it manifests makes some sense: most of the area is thickly populated with trees and plantlife, it's implied the "fog" that obscures your vision in some places is actually thick humidity, exploring further you can find cracks in the ground with pockets of lava, and there's one area where you can find that Starlight Isles sits on top of an active volcanic pit. Volcanic soil like the kind Starlight Isles would have is actually surprisingly fertile. In Undertale, monsters were probably able to get the plantlife they did Underground because "maaaagic", which to be fair is probably the same here too to an extent (especially with the mushrooms having a special magic effect), but at least all the diversity in all the magic vegetation in Starlight Isles makes sense with the richer soil compared to Snowdin.
like the kind Starlight Isles would have is actually surprisingly fertile. In Undertale, monsters were probably able to get the plantlife they did Underground because "maaaagic", which to be fair is probably the same here too to an extent (especially with the mushrooms having a special magic effect), but at least all the diversity in all the magic vegetation in Starlight Isles makes sense with the richer soil compared to Snowdin.