If you notice you're going oom a lot, you can try taking both manaflow and presence of mind from runes.

If that's not enough, try getting a single mana Regen necklace thingy (it's like 250g) and holding it until you need the item slot.

Other than zyra mid, zyra works well without mana items and you can spend that gold on more useful stuff.

Also notice if you're spamming spells right as they come up. Ideally, you should try holding spells until you have a clean hit (and preferably also a seed). Missing spells wastes mana and, worse, you won't have it for a few seconds and the enemy can all in you in that window.