Let's Talk About the Weather.

Using Some Data Toward More Productive Gardens

cherry tomato flowers - smaller blossoms suffer less blossom drop in extreme temps

cherry tomato flowers - smaller blossoms suffer less blossom drop in extreme temps

Our family moved from West Chester, PA to Raleigh, NC in 1992. Our first garden in Raleigh was planted in 1993. This past season, 2019, is therefore our 27th garden here. We had 6 gardens in Pennsylvania, with those that focused on heirloom tomatoes from 1987-1991. Using historical temperature data, I wanted to do some analyses and comparisons, as well as identify some trends, in order to confirm my observations about how gardening has changed for us here since we moved in, and to see if I could correlate the superior tomato yields achieved in Pennsylvania with the most modest results obtained here in North Carolina (accepting that temperature was only one of many variables).

One major difference between gardening in PA and NC showed in tomato productivity. In PA, I easily averaged between 20-30 pounds of fruit per plant across the many varieties I grew, approaching 40 pounds in a few cases. The plants were grown in the ground, staked, and only minimally pruned in soil augmented with mushroom compost.

In NC, yields are far more variable, and though I didn’t collect specific harvest weight data, the typical yield per plant was more in the 10-15 pound range, with occasional 20-25 pounds per plant sprinkled across the many gardens planted here. About 10 years ago I moved from in-ground dirt growing to combinations of straw bales and containers in my driveway (to take advantage of increased sun exposure there). This past year was one of the best in recent memory, with the straw bale indeterminate plants achieving the 20-25 pounds per plant range. Disease is far more of a problem in NC than it was in PA, and my yields here are often impacted by sick plants. I’ll talk more about disease in a future post; right now I want to focus on relative temperatures experienced each summer.

The reason for my interest in temperature trends is toward continually improving my own garden results, as well as being able to share pertinent findings with my fellow gardeners so that they can succeed as well. It goes without saying that my main interest throughout my near 40 years of gardening has been tomatoes, particularly heirlooms, especially those with large fruit potential. As many tomato enthusiasts know, when it gets really hot and humid (90 degrees or above, high relative humidity), fruit set becomes a problem with many varieties, leading to blossom drop and disappointing yields. By understanding what sorts of temperatures we can anticipate around which particular dates, adjustments can be made to variety choice and seeding and planting time so that extreme weather can be best avoided. Of course, our increasingly hot summers in Raleigh (as shown by the data below) make it impossible to completely remove the problem. However, there are tactics that can be used to increase success.

larger blossoms, meaning larger tomatoes, which dislike heat and humidity with regard to pollinating and fruit set

larger blossoms, meaning larger tomatoes, which dislike heat and humidity with regard to pollinating and fruit set

I’ve collected and analyzed a lot of temperature data, using the Weather Underground website and the ability to view high and low temps by month and year. Here are some of the key findings.

Extreme temperatures (temps at 90 degrees and above)

  • Between 1992 and 2005, there was an average of 36 days of 90 degrees or above.

  • Between 2006 and 2019, the number of 90 degree or more days averaged 50.

  • The summers of 2010 and 2011 had 82 and 63 days of these super hot days, respectively. In the last four years, we range between 50-60 of such days.

  • Taking it in smaller bites, the 90 plus degree days each summer averaged 36 days from 1992-2000 and 46 days in the spans 2001-2010, and 2011-2019.

I then looked at “heat waves” - spans of 4 days or more of temps at 90 degrees or above. The data was pretty surprising, but there is a clear trend. It turns out that 3 years were blisteringly hot; the summer of 2010 had 11 such heat waves, and 2002 and 2011 had 9 stretches of extreme heat. The other stretch that bucked the clear trend are the three years spanning 2012-2014, which averaged only 2 heat waves per summer.

Looking more broadly, however, it is getting hotter. Between 1992-2005, there were 45 heat waves, whereas there were 73 between 2006-2019.

Timing of heat waves

2019 was a really challenging year for heat waves (so it was not just my imagination as I was melting in my driveway during my daily 3-4 hours out there!). Each month had a long, broiling stretch - 1 each in May, June, August, September and October, with 2 in July.

2018 had 2 heat waves each in June, July and August. 2017 had 3 in July and 2 in August. Looking at the other end of my study range, 1992 had 1 heat wave, in mid-July; 1993 had 1 each in June, July, August and September. 1994 had 2 heat waves in June. In 1995 we experienced 2 in July and 2 in August. 1996 had one heat wave in June, 1997 had 1 in July and 1 in August.

Given the data, what is the best way forward for planting times and methods?

This is where it gets tricky. Increasing warmth over time would be expected to mean last frost dates moving more toward early and mid March, which would allow for a month or more earlier planting without fear of crop loss. Looking at the last frost data doesn’t show such a clear solution, though an overall trend is certainly there.

Between 1997 and 2007 (missing low temperature data prior to 1997 meant leaving out of this assessment), safe plant out dates tended to late March or early April. Between 2009 and 2019, many seasons provided opportunities for an earlier planting. Last frost dates fell into the first week of March for 2009, 2010, 2012, 2015, 2016, and 2019.

This data is particularly important for gardeners who, like me, have no greenhouse. Being able to put plants out as early as possible for hardening off or planting is critically important. Moving dozens of flats to the safety of the garage is no fun at all.

The solution? It’s time to try staggering plantings

Even though it seems like a long time span, looking at 28 years of temperature data is still pretty insignificant in the grand scheme of things. Trends can certainly be identified, but season to season variation indicates that it is not yet time to count on the increasing trend to more, hotter days and heat waves to lengthen our growing season and allow for earlier planting.

We can take advantage of the long growing season (let’s say that it runs from, on average, March 10 until November 10; that’s 9 months, or roughly 270 days!) by not putting all of our eggs in one basket - rather than planting everything as soon as we can, aiming for planting in spaced out groupings. For example, if I am going to end up growing 60 tomato plants, rather than plant all 60 as soon as the threat of frost has passed, plant 20 initially, then wait a month and plant 20 more, wait another month and plant 20 more. What this will hopefully accomplish is to avoid a series of long heat waves leading to blossom drop and significant yield reduction on all of the large fruited varieties that are planted. Staggered planting means staggered growing and staggered flowering; the odds are that favorable weather at various points of the season will lead to better overall yields.

The other consideration is in matching crop types with expected temperatures. Potatoes, peas, lettuce, beets, other types of greens (aside from Chard) are not happy when it gets hot. If we are seeing more and more 90 degree days in May, the season for such sensitive veggies becomes extremely short. Very early germination - in some cases, even planting in the fall and providing some cover protection over the winter - and planting out in late winter is the best bet in getting decent harvests from heat sensitive crops.

Let’s keep the discussion going

I’d love to hear what people think about the above, particularly in looking at real data and pondering the possibility that it had significant impacts on their gardens. I encourage people living in other areas to do a similar analysis - collect some temperature data for your area over a few decades and see what the trends are telling you. Think about how you may have to alter your timing in order to bring success back to your gardening, if changing temperatures have been negatively impacting your efforts.

Share your views via email, or as a comment to this blog. I’d love to know what you think!

Larger flowers that are indicative of the beefsteak heirloom types, though some can even look more like marigolds. Heat waves lead to increased blossom drop due to difficulty in self pollination of the blossoms

Larger flowers that are indicative of the beefsteak heirloom types, though some can even look more like marigolds. Heat waves lead to increased blossom drop due to difficulty in self pollination of the blossoms