Usually the seed packet will tell you. Poppies, sweet peas, bachelor
buttons, larkspur, alyssum, linaria, candytuft and many other cold-hardy
annuals can be sown now - in fact, could have been sown in March. On the
other hand, marigolds, zinnias, nasturtiums, four- oclocks, and most other
hot-weather late summer blooming annuals should be sown around the time of
the last expected frost. (They will usually not germinate until the danger
of frost has passed). Some seeds of warm weather annuals will rot in cold
soil, so it is important that your soil is starting to warm up too.
It depends. How's that for an answer? :-)
First of all, I'd poke around http://www.garden.org/home and read about each
of the things you want to plant. Some plants will put up with more weather
nonsense than others. Specifics are learned through reading, but more
through observing what goes on around your property in terms of
mini-climates. For instance, if the foundation of your house gets sun on the
south side, the area can sometimes behave like it's a zone warmer (at both
ends of the growing season) than a different spot on the property.
Think about whether it's practical for you to build some sort of easily
removal "tent" for covering seedlings when frost is expected. Take a look at
this for a general idea. http://www.westsidegardener.com/howto/cloche.html
An easier solution is to buy a roll of 6 foot fence wire, preferably the
plastic coated kind (for rust resistance). It's $50-$60 for a huge roll, if
I recall, but it's endlessly useful. If you bend it into a long rectangular
tunnel shape, it's rigid enough to lift off the flower bed easily without
flopping all over the place and knocking over plants. Cover it with plastic
and you've got an easy way to protect plants from frost. Just remember to
remove it before the sun's on it for very long, or you'll cook the
seedlings. Home Depot sells large tent stakes which are useful for holding
the wire structure down on windy days. Under two bucks each, if I recall.
Finally, consider the mechanical damage which can affect some seedlings. For
instance, I would never start coleus, impatiens, browallia or other really
tiny things outdoors. The sprouts are almost microscopic. Too easy to miss,
too hard to transplant (easier sitting at a table).
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