Knowing When to Plant

The tomatoes are more than ready to move up to a larger size pot.

Some of those 250 onions I started from seed....

The basket of seed potatoes. Kennebeck variety.

Knowing when to plant your garden is, of course, of great importance. Plant too early, and a late frost could kill the tender seedlings you have worked so hard to get ready. Plant too late and you have lost valuable growing time, plus cool weather varieties might bolt to seed quickly in warmer temperatures.

You can talk to neighbors and the folks at the garden center... They will likely be able to tell you the conventional wisdom for the planting dates in your area.  

Online resources will tell you what your gardening zone is, and will give you the average dates for the last killing frost of the spring, and the first killing frost in the fall.  Here are a couple of resources, though most of you already probably know this.

http://planthardiness.ars.usda.gov/PHZMWeb/Default.aspx

http://davesgarden.com/guides/freeze-frost-dates/#b

Here is one I like even better.
Spring and Fall Frost Dates

This calculator will not only give you the last and first freeze dates for your area, but will also calculate the dates for 36 degrees and 28 degrees.  And, even better, the probability, of those temperatures occuring on a given date.  Why is this helpful? 

Lets use the calculator for my area as an example. (If you click on the image, you should get a larger view.)

The town on the chart closest to us is Milford.  The last day where we might expect to see 32 degrees, our last frost date, is May 31.  Ask anybody around here and they will agree.   If you want to play it really safe, wait till then to plant.  According to this chart, there is only a 10% chance we will see any frost past that date.  

But look further.  There is a 50% chance that the last time we will see 32 degrees is two week earlier, on May 14.  Maybe I want to take the chance, especially if it is just a few seeds I might lose.   

Here is another way to use this information.  I want to plant my seed potatoes.  I know it will take about 2 weeks for the potato sprouts to poke through the soil.  So, planting two week early, on May 14, makes good sense, since by the time the sprouts actually show up, my probability of getting a killing frost is pretty low.  (And there is always a quick mulch to protect tender sprouts, or some row cover.

Also, many early spring plants, such as spinach, can withstand slightly below freezing temperatures.  A row cover, or quick hoop, usually give 3-4 degrees of protection.  So it might be worth risking planting a little early, knowing that my chances of getting that 28 degrees after, say, April 30, are only 50%, and i can protect the young plants easily enough.

This chart gives me much more information to work with.   

Couple these calculations with good old observation of the overall weather conditions and forecasts near planting time, and you can get a good idea when it is really time to get that garden started.