Set the seed flat in a warm, brightly lit area out of direct sun. The best temperature for successful germination is between 68 and 86 degrees F. The top of your refrigerator or above a hot water heater are good locations provided there is enough light available. Your portulaca seeds will germinate in 10 to 15 days.tep 2. Sow a single portulaca seed in the center of the soil in each cell of the flat. Don't cover the seeds with soil because they require plenty of light for germination.