Starting onion seed has always been a hit or miss thing for me. A few years back I tried starting them in diatomaceous earth and found much better results as far as getting them to germinate but I used fairly shallow containers which was a mistake. I now use deep pots and fill from the bottom to within an inch of the top with medium grain DE and then use the smaller grain DE for the last inch. I saturate the DE with a weak liquid fertilizer mix and then plant the seed. With this mix I have to start applying fertilizers a week or two after germination which seems to depend upon the seeds I get. Sometimes one variety will germinate great and another will not so I assume it is mainly the freshness or viability of the seeds and the temperature in the greenhouse at the time. I usually start seed in late or mid October through late November depending upon the weather here.
The advantage of the deep pot and the looser medium size DE in the pot allow the roots to develop well. It also allows for easier separation at plant out time. I will cut the tips down about two weeks before I plan to set them out and give them a good dose of fertilizer right afterwards to help them recover from the cutting. Then I try to let the pots dry out as much as possible before removing the sets and separating them. This gives me a large root structure and a shorter top so when planted they remain upright and are much easier to handle. I have found over the years that much of the top will die back after planting so cutting it off before planting doesn't slow growth once in the ground. Once the seedlings are planted and mulched with pine bark fines I will apply another dose of fertilizer and in a week or so they are growing well. The pine bark fines serve several purposes. The fines hold the plants upright after planting, they also helps maintain soil moisture, and they also reduce splash back from the soil which can lead to fungal diseases.
Down here only the short day onions work well for me. I have tried the intermediate day onions but with very limited success so I know long day onions would be a disaster. I have had my most consistent success with Bermuda onions, Granno or Vidalia type, Red Burgundy and also Creole. I find the Creole onions to be the most successful of the red onions but they are very small and pungent. Bermuda onions have been my best keepers of the white type onions but the 1015 Granno keeps good for that type.
Bill
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