Can I plant rhubarb next to cabbage?

Yes. Traditional companion pairing that makes good use of the space around rhubarb's large leaves.

Can I plant rhubarb next to onion?

Yes. Alliums planted nearby are traditionally paired with rhubarb without competing for space.

Rheum rhabarbarum

Spacing: needs about 9 square feet per plant. Mature height: 2-3 ft.

Cabbage: Traditional companion pairing that makes good use of the space around rhubarb's large leaves.
Onion: Alliums planted nearby are traditionally paired with rhubarb without competing for space.

Nothing it needs to be kept away from.

Can I plant rhubarb next to cabbage?: Yes. Traditional companion pairing that makes good use of the space around rhubarb's large leaves.
Can I plant rhubarb next to onion?: Yes. Alliums planted nearby are traditionally paired with rhubarb without competing for space.

Companion planting for Rhubarb: which plants to grow next to it, which to keep apart, and the reason behind each pairing.