My alliums are done flowering, and the stalks look like straw. Can I spray-paint the flower heads now, or will it damage the plant below ground?
There are more natural and nontoxic ways of sustaining colorful garden interest. They involve planning and selecting plants that flower at different times throughout the year. Gardens are, of course, constructed landscapes and not the same as nature. Artificially colorizing spent plant matter appeals to some gardeners’ aesthetic sense but not to others. In my own garden, the alliums are done flowering but the nearby Eryngium (sea holly) is just taking off, dotting the bed with silver and blue. Some plants provide interest even when they are desiccated and brown through the winter, and I would include allium in this group.
However, if you would like to spray color on your allium heads, there are directions here that show how to make a kind of protective dropcloth out of a heavy paper plate with a notch cut into it.
It would be good to avoid getting any paint on the ground, and I suggest not doing it on a windy day. See if you can find spray paints that have lower volatile organic compounds. Despite marketing claims, there is no such thing as a non-toxic aerosol paint as this archived article claims.
There does seem to be a trend in marketing artificially painted living plants, such as succulents which are now being sold in big-box stores. This seems a pity, since there is such beauty in the actual shades of plant foliage. Additionally, this is short-sighted, because the plant will be unable to photosynthesize and transpire, and will eventually suffocate. See the following articles from Better Homes and Gardens, and Southern Living)
Another thought: if you want to cut the allium stalks and use less toxic water-soluble paints (such as casein) to paint them and display them indoors (where rain will not wash away the coloring), that is also an option. My personal preference is to appreciate the natural color of plants as they age and decay, but again, this an aesthetic choice—just like dyeing one’s hair.