Ideally, you want to start the seeds as early as possible so that you can maximize the amount of time you get to enjoy your blooms. But if you plant the seeds too early, it is possible to stunt growth of young seedlings or kill them altogether. The best thing to do is look on the packet or look up a grow guide for the individual species you are growing. This will give you an idea of the seasonal conditions that are optimal for the given species you are trying to grow.
But let's talk about some general terms. If you are dealing with flowers such as poppies you actually want to start them very early in the season. I can say from experience that cosmos and leonotis nepetifolia (klip dagga) are not flower species that will be hurt from planting too early. You will see many packets or grow guides talking about starting the seeds as soon as the ground is workable. In this case, it means that once the ground has thawed enough that you can work in it, you can plant your seeds. Again, these are seeds that can tolerate and may even benefit from cold or frosty conditions. So when you talk about the time that this occurs, it goes back to being dependant on area.
It seems that the most common recommendation you will run into is plant the seeds after the danger of frost has passed. While the seeds themselves may be able to withstand frost, these are usually plants whose seedlings are frost tender. Many seeds in nature have mechanisms to prevent them from germinating all at once. Packaged seeds have a tendency to produce quicker and more regular germination to please the customer. But it removes the plants defense from the "irregular" nature of weather. So if you were to plant a species whose seedlings are frost tender too early, you may end up with a spout of nice weather that causes most of your seeds to germinate only to experience a period of frost that kills your seedlings. So if you pick a time in which you know the danger of frost has passed to plant your seedlings, you can avoid this problem. In many areas, May is suitable for this. But in other areas you might even wait til early June. On the other hand, you will be able to plant earlier than May down south.
But all of this speaks about planting outdoors. Many growers get a head start on the season by starting seeds indoors or in something like a greenhouse. Again, this will affect the question of what month you can plant. If you have suitable conditions, you can technically start plants or flowers in any month, and you can grow just about any species (even tropical) plants in your location. But the key to this is being able to provide suitable conditions. Indoor light simply cannot match the light outdoors. Even a well-lit window probably has less light than a shady area outdoors.
When you start your plants indoors you have to consider two things. The first is whether the conditions are suitable to maintain the plant long enough to get it outdoors. Seedlings tend to require lower light than established plants. This makes sense not just because seedlings are smaller and have less biomass to maintain. Seedlings often get their start beneath leaves, branches or under the shade of other plants. As they grow higher, they eventually find the light they need to maintain adult growth. It seems that nature understands this and has made seedlings adaptable. If your seedlings start out very tall and skinny, it is likely that your house lacks the light even to maintain seedling growth. For this, you would want to supplement the light with a grow bulb or compact fluorescent. But imagining you get past this, you do not want to start your seedlings so early that you pass the seedling stage before you can get them outdoors. So even indoors, the time of when to plant can be dependant on where you live and when the weather is warm enough outside. But it will give you a head start. As for determining when indoor planting is right, you can often look on the seed packet or in a grow guide for help. Of course, if you supplement the natural light in your house with something like fluorescent lights, you can often determine yourself when you want to plant because you will have sufficient light to keep the plants going beyond what natural light alone may offer.
The second thing you want to consider about starting seeds indoors is called hardening off. Essentially, this is the idea of getting your seedlings used to the harsh outdoor world. As mentioned before, the sun will be stronger outdoors. Just as people tend to get burned easily early in the season after lack of sun exposure, so do plants. To get your plants used to these conditions, you want to gradually expose them to the outdoors over a period of time while also increasing the amount of light you expose them to until they reach their final growing place.
Here is a basic plan for hardening off: Start them off in complete shade for a few hours a day. Over the next few days increase the time the plant it outside until it is outside all day. Monitor the plant for any signs of stress. Then eventually you want to gradually increase the light exposure until you find that your plant can tolerate its final growing place.
Some packets and grow guides will actually mention starting seeds indoors x number of weeks before the last frost. But they do not explain much of what I have just described. This will help you in such cases. Starting flowers from seed can be a bit more work than buying plants. But with seeds you will usually be able to get a lot more flowers and a lot more variety for the same price. Each year your skill will improve so that growing from seed will be second nature. But if you have any reservations about your ability when starting out, go for easy-to-grow varieties like Grandpa Ott's Morning glory, cosmos, klip dagga, California poppy, Purple coneflower (echinacea) or sunflowers. You can find seeds for most of the locally, but we also carry them at World Seed Supply. Perennials such as echinacea will return each year while some like Grandpa Ott's morning glory have a tendency to reseed. So you initial efforts will pay off in the long run. Don't be discouraged. Happy planting!
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