Hi Jalapas,

Mirabilis jalapa is an old favorite. I remember them as a kid, but I am no expert on them, so I am going to rely on the book, The Complete Guide to Saving Seeds, by Robert Gough and Cheryl Moore-Gough, for my response. It says that Four-o'clock flowers are cross pollinated by insects. So I think that answers your question,

"...could I plant some of the multi-colored flowers with my fushia ones without fear of them crossing?"

as a "no", there would be a possibility or probability of them crossing.

The book gives an interesting Plant-Breeding Hint for Four-o'clocks: "The flower color inheritance in four-o'clocks is complex. Striped varieties are all heterozygous and segregate to self colors and striped in F2. Therefore, if you plant the seeds from F1 striped varieties, some of the offspring will bear self colors and some will be striped. Tall is dominant over semidwarf and dwarf. Semidwarf is dominant over dwarf."

I understand you wanting to preserve your grandfather's strain of four-o'clocks. However, even if they did inadvertently cross with another four-o'clock, its genes would still be "in there" and capable of reappearing in subsequent generations. By allowing crosses to occur, or actually doing them yourself, you could get your grandfather's strain in some additional self colors.

ZM
(not associated with any product or vendor mentioned or linked)