It’s pretty widely known that zinnias (unlike the cosmos discussed in the post here, for which it’s never mentioned) are best when direct sown.
That’s all well and good where you’ve got the length of season to get away with it, but way down here where that’s not assured (low latitude, and a bit of altitude – bad combo), you’re sorely tempted to stick some seeds in a pot in mid-spring, and protect them from the coldest o’night temperatures to see if you can get a head start.
So far I haven’t made this work for me. But admittedly, there’s more than just root disturbance issues to get in the way. This year I sowed the seed early. I wasn’t to know, of course, that we were going to have a cold spring, and even a cold start to summer, so it turned out that I wasn’t really able to keep the plants moving through the processes of potting up and planting out as smoothly as they really need.
The literature tells us that they really won’t grow until the temp minimums reach about 10C (which is slightly different to being frost tender, though the distinction is never made) and they don’t like sitting around waiting. This may have been as big a factor in the relative failure of these plants as the inevitable root disturbance upon planting. Who knows.
But having sown them in early October and grown them on, I planted them out in early December, and at the same time sowed some seeds directly into the soil around them. Within about a month, you could barely tell the difference between them size-wise. The direct-sowns quickly took over for size, while the earlier-sown plants started to flower much sooner.
The direct sown plants just continued to get bigger and fatter, and once flowering finally started, produced much larger flowers. They also look like they’ve got loads more flower-power in terms of branching power, and therefore more flowers to come. Conditions since planting have continued to favour the self-sown plants – particularly the outrageously hot dry few months we’ve had, and the total lack of water I’ve had to give them. The total integration of their root systems with the local conditions seems like a great advantage when it gets tough.
But comparisons aside, I’ve really gotta doff my hat to these zinnias and their resilience in the face of relentless heat and absolutely no water.