Seems odd to me that there's an alert for nearly everything else, but gardening doesn't have one. Sometimes, life happens and it's hard to remember to water plants 23 hours after planting.

I agree that it's a bit odd, but the gardens are super old. They were released a long while before alerts were added. The gardens are due for an eventual revamp, though! They scored second in the old community "funded" poll results, so an alert might be added as part of the revamp.

Even outside of a revamp, there's a suggestion for the gardens getting the ability to set an alert in The list of usability and random improvement 2.0 topic. Similar topics pop up from time to time in suggestions: Older one, February this year One from July this year

So it's definitely not just you!