Documents: Hot Horticulture Issues:

CLIMATE CHANGE IN THE GARDEN: TOO MUCH WATER
by Leonard Perry
by Leonard Perry

email: lpperry@uvm.edu

In extension I serve as an advisor and consultant to the greenhouse and nursery industry, primarily in Vermont but throughout the region and beyond as well.

I give presentations on my research to the industry, and to home groups. In Research, my focus is "herbaceous perennial production systems".

His website is at http://www.uvm.edu/~pass/perry/index.html  Leonards zone of gardening: home with my trials, generally USDA 4a. Campus in Burlington is 5.


April 28, 2019

No matter where you are in the country, or whether a gardener or not, one climate extreme that is becoming more frequent is that of too much water. The saying that “when it rains, it pours” is, unfortunately, becoming more applicable now than ever. There are several options with garden practices, plant choices, and landscaping elements that will help your plants survive. Even if you experience a severe flood, it’s important to know how to recover your landscape and gardens.

Precipitation events (heavy rains) have become stronger, and more frequent. The Northeast led the country with a 71 percent increase in the most intense heavy rains from 1958 to 2012 (glisa.umich.edu/climate). The Midwest was second, with a 37 percent increase. During these heavy rains, since 1991 in these regions over 30 percent more rain has fallen that during such rains between 1901 and 1960 (nca2014.globalchange.gov/report).

The amount of the country experiencing extreme single-day rains has increased from 10 percent of the land area in 1910 to around 18 percent now (www.epa.gov/climate-indicators). These are only a few of the many studies illustrating that we’re now having to deal with more water than in the past.

Projections show that by the end of the century we may see 7 to 14 percent greater rain and snow, the higher figure under higher emissions. Yet, at the same time, we’ll likely see more short-term droughts between rainy periods—another extreme to deal with. Much of this precipitation increase is predicted to occur in winter, ranging from 11 to 30 percent more than now. More rain or mixed precipitation and less snow is predicted for winters, which will influence overwintering of perennials, among other impacts. This loss could be one quarter to one half of our current snow-covered days.

Snow is one of the best protections in winter for herbaceous perennials. Less snow may lead to more plant losses, and actually the ability to grow fewer perennials than now in areas with sufficient and reliable winter snow cover. Overall snow cover in the Northern hemisphere, particularly the far north, has declined each year since 1986 except one, with a steep decline since 2003 (Rutgers University snow lab).

Consequences of too much water include springtime flooding delaying planting; root damage and reduced yields; soil loss from erosion and silt deposits when land floods; and contamination of water from runoff. So, what can gardeners do, in addition to getting a good pair of boots, to prepare their gardens and landscapes for more water? These tips are particularly relevant if you have soils or areas that tend to stay wet and soggy after downpours, or that may even flood periodically.

Choose tolerant plants for wet areas. While few plants tolerate permanently wet soils (except water and bog plants), Siberian iris, joe pye, turtlehead, foxglove, ligularia (shade) and astilbe (shade) are some perennials for wet soils, the latter two preferring not to dry out. River birch, hackberry, green ash, swamp white oak, pin oak, willow, and bald cypress are some trees for wet soils.

Red chokeberry, summersweet clethra, shrub dogwood, winterberry, and purpleosier willow are some shrubs for wet soils.

Use raised beds to grow above the wet soil; the longer the soil stays wet, the higher the bed should be (one foot or more). Grow shrubs or trees on slight mounds.

Avoid working on soils while wet, as this will destroy soil structure. What’s too wet? A ball of soil in your hand should hold together, but crumble when pressed and not ooze water.

It’s hard to add too much organic matter, such as compost, to soils, particularly if they’re sandy or gravelly. In addition to helping the soil dry out more quickly, organic matter improves soil physical properties and helps feed beneficial soil microbes. Minimizing, or even avoiding, soil tillage (using a spading or broad fork to loosen soil is better) preserves soil structure which, in turn, helps it to recover more quickly after heavy rain.

Incorporate drain pipes or tiles to help remove water from areas if they are the only choices for planting, and there is somewhere to redirect excess water.

Reduce stormwater runoff from paved surfaces by using permeable pavers and surfaces. These work on surfaces that don’t slope more than one foot over a horizontal distance of 20 feet. While you might create such walks and patios yourself, a professional landscaper is best for large surfaces such as driveways made of permeable pavers.

If you have steps up a slope, or need to make some, consider permeable ones for infiltration. Create rain gardens as a destination to hold water from heavy rain events, allowing it to percolate back into the soil over time (www.groundwater.org/action/home/raingardens.html), or vegetated swales to treat water flowing through an area.

Even simpler than a swale is a diversion ditch or channel, filled with gravel or pebbles. These are what often are placed under the dripline of roofs. Use attractive large pebbles or river stones to create an attractive creek bed feature in your landscape.

Another runoff option might be to simply dig a small pond where water can flow during a heavy rain, even if the pond doesn’t have water all year. Such catchment ponds sometimes are seen near parking lots to catch runoff.

Use rain barrels or similar holding tanks to collect water during heavy rains to use later. Many prefer not to use such runoff from roofs on edible crops, as it may contain pollutants.

Consider installing green roofs on sheds or garages to slow and reduce runoff.

Avoid removing too much vegetation from slopes, to avoid erosion during heavy rains. If you have a fresh slope, add vegetation such as grasses or cover crops. If you want or need to plant them, consider terracing using wood, stones or hardscape materials if the slope is steep.

There’s not much you can do if an area is flooded except to be patient, and hope that the water subsides soon. After a flood, once the soil is somewhat dry, remove it from beds and around plants, wearing gloves if pollutants from elsewhere may have been brought in. If any plant parts were underwater, wash them off with the hose.

Watch for signs of diseases; also watch for nutrient deficiency, fertilizing as appropriate or using foliar feeding (spray fertilizer onto leaves). For edible plants, destroy greens, produce eaten raw, and any other vegetables near to harvest.

Wait until next season to grow crops, that are to be cooked, on that site. Wait two seasons to grow salad crops or those to be eaten raw, so potential diseases can leave the soil.

Don’t forget your container plants during heavy rains. Having an organic, well-drained potting mix goes a long way to helping containers dry out quickly. Of course, with advance notice of incoming rain and if containers are easily moved, consider putting them under shelter.

If containers are in an area that floods and they’re under water, or sitting in it a while, carefully remove the plant from the pot and let the roots dry out in an area out of direct sunlight, and with good air movement. Wash the pots well and, if plants are still relatively small, repot them into some fresh and dryer potting mix. If you have vegetables in containers that were flooded, use the same precautions as those grown in field soil.

While it’s difficult to fully recover a landscape or garden after heavy rains and flooding, these preventative measures and choices will help your plants to better cope. You’ll also be minimizing the negative impacts of water runoff. Many weblinks to resources on reducing rain runoff are available from the EPA (www.epa.gov/soakuptherain).

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