Say I cast the Speak with Plants spell, targeting a plant creature, and during the duration, I cast the Geas spell on the plant. Can I continue to control the plant after the Speak with Plants spell ends?
"You place a magical command on a creature that you can see within range". Plants, like humanoids, monsters, and undead, are creatures, so they are valid targets for the spell. You can only give the initial command while the plant can understand you, but after that, it will follow those commands or take psychic damage, and will be "charmed by you for the duration". You will be unable to issue additional commands without casting Speak with Plants again, as "a creature that can’t understand you is unaffected by the spell".
Geas will continue to have an effect.
Here's the first few sentences of geas, with my emphasis:
You place a magical command on a creature that you can see within range, forcing it to carry out some service or refrain from some action or course of activity as you decide. If the creature can understand you, it must succeed on a Wisdom saving throw or become charmed by you for the duration. While the creature is charmed by you ...
The first sentence defines what the spell can target: a creature that you can see within range. Any creature, including a plant that qualifies as a "creature", is a valid target, provided that you can see it and it's within the 60-foot range.
The creature does not need to understand you to be a valid target. You can geas a moose if you want, and the spell will take effect.
The second sentence defines what happens, and the first part of that sentence sets a condition for what happens: if the creature can understand you. This is because you may not know if a creature can understand you when you cast the spell! If you geas an orc and issue commands to it in Old Toonish, you're risking the spell having no effect because the orc grew up in the southlands and can't make head nor tails of your Toony accent.
If the creature can understand you, and it fails a saving throw, then the creature is charmed, and the condition for the remainder of the spell's effects is defined: while the creature is charmed by you. Note that this also doesn't say anything about the creature understanding you; they just have to remain charmed (that is, having the charmed condition) by you.
So: If you cast speak with plants:
You imbue plants within 30 feet of you with limited sentience and animation, giving them the ability to communicate with you and follow your simple commands.
If you then geas one of the plants (assuming your DM agrees it's a creature), then the plant can almost certainly understand your commands and it may be affected by the spell if it fails a Wisdom save.
However, when speak with plants wears off (after its duration of ten minutes), the plants will lose their sentience and animation; depending on the nature of your command, they may be incapable of obeying your instructions, and thus may take psychic damage, etc. The geas is still in effect; it's just that the plant isn't any good at following the command you gave it.
Geas is probably suppressed, but ask your DM
There's no rule governing what happens when a valid spell target temporarily becomes an invalid target. A good rule of thumb is that the spell is suppressed while the target is invalid.
This suggests that, since the Geas spell requires the target to understand you, then the spell is completely suppressed after Speak With Plants ends. However the spell is only suppressed, not removed, so if you cast Speak With Plants again or otherwise reestablish communications before the duration of Geas runs out, it will come back into effect.
However, in that same quote from Crawford, he notes that there isn't a hard rule here. Your DM could just as easily interpret it as "it keeps following your orders" or "the spell is canceled". I'd suggest talking to your DM and figuring out beforehand how this will work if you plan to use this tactic in-game.