Learn Danish: Frøet vokser under jorden. - The seed grows under the ground.

It is, but it is a tricky double-meaning! Danish has et frø (neuter) which means a seed, making its definite form frøet (the seed). It also has en frø (common gender) which means a frog, making its definite form frøen (the frog). The -et ending here tells you it is the seed.

Could I use gror instead of vokser in this sentence?

Yes, Frøet gror under jorden is also perfectly natural. At gro is specifically used for things that sprout from surfaces like plants, hair, and nails. At vokse is a broader word for increasing in size, used for plants, children, and abstract things like a growing economy.

Does jorden mean the ground or the planet Earth?

It can mean both! The noun en jord can refer to soil, dirt, the ground, or the entire planet Earth. Context tells us which one it is. Here, because a seed is growing in it, it translates best to the ground or the soil.

Would it be wrong to say i jorden instead of under jorden?

Not at all, both are correct and common. Under jorden emphasizes that the seed is hidden beneath the surface. I jorden just states that it is physically planted in the soil.