It's better to use something soft, with a larger surface area than twine, especially if the plant has herbaceous (non-woody) stems and the tie will be supporting the weight of the plant. Twine would certainly damage the stems.

What to do, then?

A. You can cut soft fabric, such as that from an old T-shirt, into strips of about three to four centimetres wide.

B. Some flower shops sell special plant ties. They also carry little clips that are especially convenient for orchids.

C. Usually, packs of garbage bags come with special green ties (other products, too). They make excellent ties for plants, but you should arrange them loosely; leave enough space between the stem and the tie for the plant to be comfortable. Use two ordinary elastics to tightly close your garbage bag.

D. Strips of discarded pantyhose are probably the best material to use as plant ties.