Archives for posts with tag: average date of last frost

You can tell that we’ve finally passed the point at which cold nights can be expected; there is a freeze watch in effect for tonight. I’m not too worried—the National Weather Service does not actually predict sub-freezing temperatures—but I will cover the east planter with black plastic sheeting just to be safe.

The radishes, always first off the starting block, made their appearance three days ago and the Sugar Snap peas, not to be left behind, started to peek out from the soil a day later. There are now seedlings to protect and the root vegetables are particularly susceptible.

With the trellis in place, I cannot fully cover the peas, but I don’t think it is necessary. The pea shoots are quite hardy and even without completely enclosing the planter, the sheeting will capture the heat that the garden acquired during the day.

I wonder what date the National Weather Service uses for last frost in our area? I conservatively use May 5, which has a 90 percent confidence level (i.e., there is only a 10 percent chance that the temperature will fall below freezing). Apparently, the NWS uses an earlier date.

I suspect that they use a lower confidence level, probably at a 50 percent chance of exceedance. Their date—whatever it might be—is less conservative from a freezing temperatures point of view but more conservative from a freeze warning point of view (i.e., its use will likely generate more warnings). Given that the NWS is in the business of forecasting the weather and not gardening, this makes perfect sense.

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Speaking of lettuce (see February 7, 2014), a quick consult of the seed sowing calendar reveals that now is the time to sow lettuce seeds for non-transplanted growing.  (The nice thing about the seed sowing calendar I developed last year is that it is relative to the assumed average date of last frost, which is essentially unchanging; see March 23, 2013.  Therefore, last year’s calendar will be just as accurate this year.)  By “non-transplanted”, I mean that we will start the seedlings indoors and then pot them up to larger containers that can be moved outdoors when the warmer weather catches up to us.

At a certain level, it seems unbelievable that we would be even thinking about planting something as delicate as lettuce at this time of year.  Especially this year:  Temperatures have been in the single digits and snowstorms are weekly events.  There is no feeling (I don’t feel it, anyway) that the wintery weather will be changing anytime soon.  And yet, we are approaching mid-February and in two weeks it will be March.  By my reckoning (see June 25, 2013), that’s the beginning of spring!  And what says spring more than fresh lettuce?

To get the lettuce plants started, I followed the same process as I did for the herbs (see January 28, 2014).  I mixed up a batch of seed starting mix (peat moss, vermiculite and perlite in a 2:1:1 ratio with a teaspoon of lime), moistened it with water and filled half of a compartmentalized seed tray.  I then planted six of the compartments with seeds for romaine lettuce and six with red leaf.  In a couple of weeks, I will plant another six of each variety followed by a final six of each two weeks after that (a half-tray has 36 compartments.

Assuming the lettuce seeds are still viable (and they should be; the seed packet indicates an average life of two years and they have been stored properly), they will sprout in seven to 12 days.  They’ll need a couple of weeks to get large enough to transplant and then four to six weeks to reach full size.  If the lettuce plants last that long (we may start eating them earlier), it will be some time in the middle of April.  Therefore, it is unlikely that the first batch will spend any time outdoors.  But the second and third sowings probably will.

I moved the half-tray of lettuce seeds onto the seed starting apparatus where it joined the herbs, already in progress.  They haven’t changed at all—their status is holding at four basil seedlings and two presumed rosemary seedlings—but I’m not worried yet (well, not too worried).  Herbs are notoriously slow to germinate (which is why we started them in January).

Up next, per the seed sowing calendar:  Bell peppers and eggplant in the first week of March.

Once again, the odds are against us.  Last night, the National Weather Service issued a frost advisory and tonight, a freeze warning is in effect.  We are more than a week beyond the official last expected frost date but as noted before, that doesn’t mean that frost or freezing temperatures are impossible, just that they are unlikely.  This late in the season, it should be very unlikely.

Have we entered a period of unlikely occurrences?  If I had a jar filled with uncooked rice and beans, would the two be perfectly separated, one layered over the other?  [See The Eyre Affair, by Jasper Fforde.]  Would now be a good time to travel into deep space?  [See The Hitchhiker’s Guide to the Galaxy, by Douglas Adams.]  Should I run out and buy a Lotto ticket?

Last night, I covered the most delicate seedlings in the garden—the romaine and red leaf lettuces—and tonight I will do the same.  But with a greater risk of freezing temperatures, I will also cover the west planter.  I should be able to get black plastic sheeting over everything except the peas who extend well above the top of the planter (even if their trellis was not in the way).  They will have to tough it out.

Even during the day it is colder outside than it is indoors (just like winter!).  Therefore, I am suspending the hardening off.  It’ll be like a snow day except that everybody has to stay inside.

Late yesterday afternoon, the National Weather Service posted a frost advisory for our area.  I didn’t see this until I checked the forecast at about 9:00 pm.  The advisory did not seem to make sense—the expected overnight temperature was warmer than 40 degrees—but just to be on the safe side, I covered the garden with a sheet of black plastic.  The effects of micro-climate should not be underestimated.

The temperature did drop into the 30s (but only just, at 39 degrees) so there was no real danger of frost.  Still, I imagine that the peas, carrots, turnips, radishes and beets appreciated the protection.  They have spent their entire (if short) lives out in the cold and probably envy their cousins (the tomatoes, squash, cucumbers, eggplant, bell peppers and basil) who have yet to venture outdoors.

Now that we are into the month of May, the risk of further frosty nights is rapidly diminishing.  It’s tempting to think that the threat has vanished but statistics don’t work that way.  No, there is still a non-zero chance of freezing temperatures but the probability has dropped below ten percent.

In preparation for planting, I sorted our seeds (for a list, see February 8, 2013 and February 8, 2013, part 2) according to sowing method and incubation period.  We’ve been guided this year by the Week-by-Week Vegetable Gardener’s Handbook by Ron Kujawski and Jennifer Kujawski.  As the title implies, the book’s advice is arranged chronologically, relative to the date of last frost, which makes it very practical and easy to use.

Our seeds fall into three basic categories:  those that will be sown indoors before the last frost, to be transplanted when the weather is sufficiently warm; those that can be planted outdoors while it is still cold (i.e., before the last frost); and those that are best planted outdoors after any significant threat of frost has passed.

Some of the plants in the last category could be sown indoors (prior to the end of cold weather) but not all of them can be easily transplanted.  For instance, transplanting individual lettuce seedlings would be a tedious business and the chances of the seedlings’ survival would be diminished.  For some of these plants (again, the lettuces), we may elect to plant them indoors in large pots and then simply move the pots outdoors when the weather warms.

For the plants started indoors, some may need to be potted up before transplanting (e.g., the tomatoes) while others may not (e.g., the squashes and cucumbers).  All will want to be hardened off before migrating outdoors permanently.

Following the book’s lead, I tabulated our seeds into a Seed Sowing Calendar.  The only vegetables not listed there are the string beans.  They will be planted in the same spot occupied by the Sugar Snap Peas, after they run their course.  Last year, this was in early June.  We did not plant until July that year but will try to turn the crop over more quickly this year.

I’ve chosen May 5 as the date of last frost and that puts us today at six weeks before.  That also puts us three weeks behind on sowing seeds for eggplant and bell peppers and a week behind for peas.  I’m not worried about the peas—they wouldn’t be doing much outside in the cold anyway—and I’m not really worried about the eggplant or peppers, either.  They are late season vegetables so a late start should not make much of a difference.

On the positive side, the time is right to plant tomatoes and basil indoors and there are several other vegetables—carrots, turnips, beets and radishes—that can be sown outside at any time now.  The lingering cold and its effect on us (not the plants) is the only thing holding back our enthusiasm.

It’s not as complicated as it might look or sound but sometimes I ask myself, what have we got ourselves into?

We’ve been starting to think about this year’s garden which is the first step towards actually doing something about it.

When planning at this early stage, the milestone that comes up most often is the average date of last frost.  Seed choice, time of sowing, period of hardening off, and date to transplant all work backwards (or forwards) from this important seasonal transition.

So, when is it?

In the old days, we would ask an old-timer or consult an almanac.  I don’t know anyone around here who falls into the former category but there are a variety of almanac websites (and, presumably, one can still buy a print version), most of which provide a list of cities and dates.  Looking at the Farmers’ Almanac, for instance, I found a map of the US and after clicking on New York, a short list of cities popped up.  The nearest to us is Albany for which the average last spring frost is May 2.

That was very simple and convenient but Albany is significantly farther north than we are.  Also, the notes indicate that there is a 50 percent chance that frost will occur on a later date, which sounds risky.  With further research, I thought I might be able to determine whether the risk of frost—with a higher confidence level—might be expected to end earlier.

Other websites attempted to refine their estimate by combining information from several nearby cities, based on our zip code.  For example, Moon Garden Calendar presented data for Poughkeepsie (to the north of us) and Yorktown Heights (to the south).  For a 10 percent probability of exceedance (the site allows a choice), the dates range between May 14 and April 29; a linear interpolation would give us May 7.  At Dave’s Garden, three cities were presented (Mohonk Lake and Middletown, both across the river from us, and Poughkeepsie) and their information was averaged for me.  The “almost guaranteed” date after which frost will not occur is May 4.

These dates still seemed late to me so I went looking for other sources.  I next tried the Cornell Cooperative Extension (CCE), which provides a map on which rough contours have been plotted.  Based on this map—which is extremely low in its resolution—we are on the border between a last spring frost in the range of April 30 to May 10 and April 20 to 30, with proximity to the Hudson River clearly accounting for the latter zone.  We are in a transitional area with a microclimate that depends not just on the river but elevation as well.  Apparently, I needed a more detailed map.

A little web searching led me to PlantMaps.  This site is a great example of a geographical information system (GIS), where a lot of available information is linked to a physical location.  In this case, after entering my zip code, I was presented with a zoomed-in interactive map of New York hardiness zones (see July 28, 2011), along with a list of other available maps, among them a Last Frost Date map.  I clicked on this link, and was directed to another color-coded contour map.

This one appeared to be in greater detail than the CCE version—after all, it is powered by Google Maps—but the problem with it is that because of all of the detail (roads, terrain, satellite imagery, etc.), the contour colors are difficult to match with the legend.  As best as I can tell, we are located somewhere between a last frost date of May 1 to 10 and April 21 to 30.  This is essentially the same result as before but with higher resolution.

Which should not be a surprise.  The source cited by all of these websites is the National Climatic Data Center, a part of the National Oceanic and Atmospheric Administration.  They maintain data for a long list of locations in each state, compile and average them, and maintain contour maps that present the information visually (the raw data is also available in text form).  Their map (which is low resolution, just like CCE’s) puts us squarely in the May 1 to 15 zone of last spring frost; the nearest city listed in their database is West Point, with a last spring frost date of May 3.  The Farmers’ Almanac had it right after all.

Based on all of this, we are approximately 14 weeks before the last frost.  Therefore, we’re already behind!  Of course, I knew that before I even started this exercise.