1A seedling or young plant — a small plant grown from seed, typically intended to be transplanted. A general horticultural term used across vegetables, flowers, and fruit trees; you'll see plants sold as 〜の苗 at garden centers and home-improvement stores.
ホームセンターでトマトの苗を三本買ってきた。
I picked up three tomato seedlings at the home-improvement store.
苗のうちは根がまだ弱いので、水のやりすぎに気をつけて。
While they're still seedlings the roots are weak, so be careful not to overwater.
ベランダで育てていた苗が、ようやく花を咲かせ始めた。
The seedlings I'd been raising on the balcony have finally started to bloom.
祖父から庭木の苗を分けてもらった。
My grandfather gave me some sapling cuttings for the garden.
2Specifically, a rice seedling — a young rice plant raised in a nursery or seedling tray and transplanted into a paddy field in spring (田植え, rice-planting). In a rice-paddy context, 苗 almost always means this. Related terms: 早苗 (early spring rice seedling), 苗代 (rice-seedling nursery bed).
五月になると、家族総出で田んぼに苗を植えた。
When May came, the whole family would head out and plant rice seedlings in the paddy.
今年は天候のせいで苗の育ちが悪い。
Because of the weather, the rice seedlings aren't growing well this year.
祖母は今でも、自分の田の苗を毎年自分で育てている。
My grandmother still raises the rice seedlings for her own paddy every year.