Colchicine is a drug. It, like any other drug or substance can be toxic depending on the dose. Even tap water is toxic if you ingest enough at one time.

Since a gout patient takes three or 4 pills (0.6mg ) the theraputic dose is 1.8 to 2.4 mg in a single day. This is direct oral ingestion.

Contrast that with a single pill (0.6mg), a fraction of which is applied to several seedlings and then exposed to intense light for some months, which completely degrades the original alkaloid.

But to be fair, the most realistic hazard would be to carelessly handle
the colchicine when applying it to the seedlings. You wouldn't want to wet your fingers with it, and wipe your eyes for example. But then I wouldn't want to get fertilizer or beer in my eyes. It all comes down to common sense.