Why ‘normal’ rainfall wasn’t enough for our 2024 gardens

On paper, the 2024 growing season appears to be an overall normal one rainfall-wise.

The National Weather Service rain totals for the six prime growing months of April through September clocked in at just more than 241⁄2 inches for the Harrisburg area – almost exactly the norm.

Even with the parched October that brought less than a half-inch of rain all month (on the month's first day), the total rainfall for the whole year comes in at 37.26 inches. That's a mere half-inch under our normal January-October total as measured since 1991.

Central Pennsylvania also mostly avoided the threatening drought warnings that so many other parts of the U.S. experienced – until last week, that is, when 33 Pennsylvania counties , including the whole southcentral Pennsylvania region, finally earned that distinction.

Yet it somehow dry most of the time this year.

Lawns had bouts of brown at times, new plantings wilted, and gardeners will tell you they spent plenty of summer hours with hose in hand.

That wasn't a delusion or a case of gardeners' favorite pastime – complaining about the weather.

The explanation is that it's not just how rain that falls that counts, it's also it comes.

When you look into those 2024 details, you'll find it was feast or famine for rain this season.

January started off rainy, giving us a nearly three-inch jump on the norm.

Come spring, National Weather Service readings from Harrisburg International Airport show that our April-September rain total was "normal," but some 60 percent of it came on just 10 of the period's 183 days. And seven of those were clustered – coming within a week or less of one another.

In between those precious few "good" rains, our yards and gardens got five long strings of little to no rain – 21 straight days from May 15 to June 4; 20 straight from June 6-25; 22 straight from June 30-July 21; 32 straight days (other than a trace) from Aug. 19 to Sept. 20, and nothing for almost all of October.

Those are all long enough to dry the root zone of plants and grass. That translates into stunted growth and maybe even plant death for young plants that just went into the ground in spring or last fall.

Dry air and hot temperatures compound those multi-week stretches of no rain.

That also happens to be the exact weather we got soon after a comfortable soil-moisture start to May.

Once gardeners had their summer tomatoes and petunias in the ground, nature's spigot shut off with that 20-day no-rain period from May 16 to June 4.

May's temperatures also ran 2.8 degrees warmer than usual, hitting 90 degrees on May 2.

June 5 gave us a reprieve soaking of 1.4 inches of rain, but then came 20 more days of no significant rainfall.

The month ran 4.3 degrees warmer than normal with nine days of 90 degrees or hotter and another nine days topping out between 85 and 90.

June ended with a 1.5-inch soaker on June 26 and another inch three days later, running the month's total above average – even though the bulk of June was functionally dry.

July got even worse. Temperatures sizzled at 90 degrees and higher for 16 days of the month, hitting 100 on July 16 and bringing the monthly average to nearly 3 degrees warmer than usual.

Meanwhile, meaningful rain didn't happen for the month's first 21 straight days (and 29 of the month's 31 days overall) before a 1.24-inch soaker finally turned the tables on July 30.

Even August's relief as the year's first month with a below-normal temperature average and 1.74 inches of rain than normal looked better on paper than on the ground.

Almost all of the rain came early in the month during three rainfalls over eight days. Despite relatively pleasant temperatures for August, we had no useful rain for 32 straight days after those three rainfalls until a Sept. 21 soaker of 2.2 inches.

What all of that shows is that when it comes to plants, what matters most is what meteorologists call "effective rainfall."

For gardeners, this is rain that happens often enough and in sufficient amounts to keep the plant root zones consistently damp. That can range from the top few inches for small plants such as annual flowers and most vegetables to one to two feet deep for shrubs and trees.

The problem with even frequent but shallow rains – say, under a quarter of an inch – is that it's only enough to dampen the soil surface or maybe only the mulch layer that tops the soil surface.

Since soil closest to the surface is generally the first to dry out – especially in hot, sunny, dry weather – plant roots can starve for moisture just days after an "effective" rain that wets the soil to a foot or more.

The soil dries even faster and deeper near big trees, whose roots mine moisture from underneath while dry air evaporates it from the surface.

Those scenarios are behind the general gardening rule that plants need one inch of water per week to thrive. If nature doesn't deliver that amount (and it usually doesn't), that's when gardeners are supposed to step in with their hoses, buckets, and irrigation systems.

Another problem with feast-or-famine rains is that dry soil and dry mulch don't accept moisture well at first. The earliest moisture has to break the surface tension before water percolates downward.

If you've ever hose-watered a dry, mulched bed and noticed the first application beading and washing off instead of soaking in, that's what's going on. Once the surface is wet, the flow soaks in much better.

This is also where rainfall intensity comes into play. If it rains so hard that an inch falls in 15 minutes, the soil can't absorb it all that fast. Much of it runs off into storm drains and creeks instead. Most soils absorb rain most efficiently when it comes at a rate of about one-tenth of an inch per hour.

That means that the same inch of rain spread out over a day would be much more "effective," dampening down to six inches or more depending on the composition of the soil (i.e. sand vs. loam vs. clay).

Texas Extension horticulturist Dr. Larry Stein says the timing and intensity of rain explains why it's actually helpful to water just before or even during a rain (when neighbors will surely think you're crazy).

But as Stein explains in a Texas Extension post, "Of course, if it's raining 'cats and dogs,' one would not want to water because the water is already coming so fast that it will run off anyway. But mist and drizzle is a different story. No water will be evaporating since it is raining, and the amount you apply along with the mist or drizzle will help wet the soil to a greater depth. If all we get is a cloud burst, the only places that will get any real benefit will be the places where the water ponds."

The bottom line is not to fret that you've wasted water if you happened to irrigate before a light to moderate rain. Unless the rain turns out to be a soaker, you've essentially "tag-teamed" with Mother Nature to give plant roots the deeper moisture they need.

The unfortunate part about this season's feast-or-famine rain is it's fast becoming the norm, not the exception.

Combined with our steadily increasing summer heat, our gardening seasons are changing fundamentally.

Instead of our traditional growing season from spring to fall followed by a dormant season of winter, we're morphing into four seasons: 1.) winter; 2.) a good spring growing period of March through May; 3.) a "survival season" of spot droughts and unrelenting heat that can run from June through August; 4.) another good growing period from late summer through October (maybe).

That's consistent with what climatologists have been predicting and observing – a shift from a "Goldilocks" climate in which temperatures and rainfall are "just right" into a more erratic climate of yo-yo-ing extremes.

An April 2024 University of Maryland-led study published in concluded that the warming planet is leading to a "year-round, worldwide trend toward more intense but less frequent rainfalls... Already in most regions, more than half of the total yearly rainfall occurs on the 12 wettest days of the year."

In other words, it isn't just us and isn't just a 2024 anomaly.

The trend has spawned a new term in climatology known as "ecological drought."

This describes our 2024 summer well in that it's when dry spells are significant enough to impact plant growth, wildlife activity, and other ecosystem functions. That can take place even when traditional drought measures such as aquifer levels, reservoir levels, and stream flows are OK.

Our gardening conditions aren't just changing in summer. Albeit interspersed with cold snaps here and there, our winters have been running much warmer – enough to make a difference in how plants respond and which plants will withstand our winters.

This past year, for example, December 2023 got no colder than 24 degrees overnight, finishing the month 5.3 degrees warmer than average.

January 2024 was 4.2 degrees warmer with a low of 14 degrees – the coldest it got all winter.

February was almost 5 degrees warmer with a low of only 20 degrees and temperatures that hit 61 and 62 degrees for three straight days Feb. 26-28.

We were done with frost (at least at the official monitoring site of Harrisburg International Airport) this year by the unheard-of early date of March 26.

That was just one week after winter ended and came a full six weeks before what Harrisburg-area gardeners consider to be the traditional "safe" frost-free date to plant summer flowers and vegetables (i.e. around Mother's Day).

The upshot is that plants are generally blooming earlier, bugs and weeds that once would've died in our winter cold are now surviving, and our frost-free growing season is lengthening on both ends.

For gardeners, that means adjusting our time frames, rethinking our plant selections, and above all else, being more ready than ever for those extreme curveballs... any time of year.

Read George's column on gardening in the "new normal"

Read George's column on how the changing climate is changing gardening