Planting before the last spring frost

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The pigs are out of the garden space and have removed all their fencing.  My seed orders are starting to roll in. The final main garden layout is ready. I’ve been running outside to weed ornamental beds in between rain showers (and now snow).

The Greendays Gardening Panel has returned to their 10:00 am spot on my local NPR affiliate station. In short, I’m chomping at the bit for my spring planting.

When I first started vegetable gardening I assumed that since the plants I was growing were annuals, and that many originated in tropical or Mediterranean climates, that I should plant everything after all danger of frost had passed. Over time, I have learned that some of the plants referred to as being a “cool weather crop” can be planted before the last frost.

No one really agrees on any of these numbers, to my immense frustration, but here’s some general guidelines I’ve compiled (mostly from Rodale’s Illustrated Encyclopedia of Organic Gardening and The Backyard Homestead, Ed. by Carleen Madigan):

  • Bean, dry: After all danger of frost has passed. Min. soil temp 50ºF
  • Beets: Two weeks before last frost. Min. soil temp 45ºF
  • Carrot: Two weeks before last frost. Min. soil temp 45ºF
  • Cucumber: Direct sow two weeks after last frost date or start indoors at frost date and transplant in two weeks. Min. soil temp 68ºF
  • Leek: Up to one month before last frost. (As soon as soil can be worked.) Or start indoors up to 8 weeks before last frost date and transplant at or a week after last frost date Min. soil temp 45ºF
  • Onion: Up to one month before last frost. (As soon as soil can be worked.) Min. soil temp 45ºF
  • Parsnip: Up to six weeks before last frost. Min. soil temp 35ºF
  • Pea: Up to one month before last frost. (As soon as soil can be worked.) Min. soil temp 41ºF
  • Potato: Up to one month before last frost. (As soon as soil can be worked.) Min. soil temp 45ºF
  • Pumpkin: 1-2 weeks before last frost. Min. soil temp 55ºF
  • Radish: Two weeks before last frost Min. soil temp 41ºF
  • Spinach: Up to one month before last frost. (As soon as soil can be worked.) Min. soil temp 45ºF
  • Tomato: Start indoors 6-7 weeks before last frost or direct sow after all danger of frost has passed. Min. soil temp 65ºF

If there’s anything you are interested in growing which is not on my chart I strongly encourage you to get a copy of Rodale’s Illustrated Encyclopedia of Organic Gardening. It really has just about everything you need to know to grow food – and it’s a wealth of inspiration, too.

[Note from the future about the following paragraphs: These NOAA products have been replaced by one interactive site you can find here.]

To find out your average last frost date visit the NOAA’s NESDIS NCDC US Climate Normals (National Oceanic and Atmospheric Administration’s National Environmental Satellite, Data, and Information Service National Climatic Data Center). This site will give you information, grouped roughly by county for your state, on the likely first and last frost dates based on data compiled from 1971 to 2000. This chart gives data for three freezing levels, 36°F, 32°F, and 28°F, at three probability levels, 90%, 50%, and 10%, which I initially misunderstood. I thought that these must indicate that there was a 10, 50 or 90 percent chance of the listed date being the last chance for the listed temperature, but it is in fact, the reverse. According to a NOAA report titled Freeze/Frost Data:

“For example, suppose the .90 probability level for the spring season is computed to be March 1 at the 32° threshold. This means that nine years out of ten a temperature as cold or colder than 32° is expected to occur later than March 1 during the spring season.” [emphasis original]

There is no NOAA weather station in my podunk town, so I averaged the dates from the two closest stations and came up with May 7 as the date after which there is only a 10% probability of 32°F weather from that date onward. That means, for example, that I can plant potatoes on April 7, but because I chit my potatoes for 1-2 weeks I will have to have them on hand by March 31 at the latest.

— Amanda

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