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How to Harvest Rainwater

Thom Morgan

Thom Morgan, Springmoor landscape manager

By Thom Morgan, Springmoor Landscape Manager
Although the North Carolina Piedmont is not currently in a drought, many of us remember 2007, when large portions of Falls Lake, our primary water source, dried up and the City of Raleigh issued mandatory watering restrictions.

Things to Consider

Cost will be a big consideration when building a rainwater storage system. Summer is mainly when we need to water our vegetables and ornamental plants. Gardens in the Raleigh area should receive about 60 gallons of water a week per 100 square feet.

Next figure out how many square feet of garden you need to water. Measure the width of the area, then measure the length. If your garden measures 15 feet wide by 20 feet long, your garden equals 300 square feet. To figure out the amount of water you’ll need, multiply your square footage by .6 gallons. (If 60 gallons is needed for 100 square feet, .6 gallons is needed for 1 square foot.) In this example of a 300 square foot garden, you’ll need 180 gallons of water for your garden.
The average cost of a rain harvesting system is around $2.00 per gallon, so for your 300 square foot garden you would spend about $360 for a barrel, down spout adapter, a tap, a cover to prevent mosquitoes, and a hose to gravity feed your water to where you need it. Figure in the cost of a pump if your rain storage system has to be placed lower than the garden where it is needed.

How Much Water Can be Harvested?

An area of roof 1,000 square feet will supply 550 gallons of rainwater per 1 inch of rain. So if you have a 1,200 square foot roof area, and you receive 2 inches of rain, you would get 1200 x.55 x 2 = 1,320 gallons. A system to hold that much water, if needed, would cost $2600 at the aforementioned estimated price of $2.00 per gallon.
Rain barrels found at most garden supply stores typically hold 50 gallons, and cost about $80.00


Ideally, you will want to locate your rain water collector uphill from your garden to take advantage of gravity. If this is not possible, a small garden pond pump or sump pump will do, if the vertical difference is three feet or less. For steeper slopes, a system that collects rain at the top of the area to be watered should be considered.


A gallon of water weighs 8 pounds, so your 50 gallon rain barrel will weigh 400 pounds. That much weight must be placed on a concrete pad, or a 14 inch thick gravel base to prevent the barrel from sinking and possibly tilting.

Top Cover

Most rain water barrels come with a cover which adapts to your downspout. The cover prevents insects, especially mosquitoes, from using your rain barrel to start a family. A cover also prevents leaves from falling in and plugging up the faucet that you attach a hose to.


Rain barrels are of very little use during the winter, so it is best to drain any remaining water to prevent freezing and cracking of the system.


If your watering needs exceed the 50 gallons most systems contain, then it is best to talk to a landscape architect, or designer that is familiar with designing rainwater retention ponds.

August Focal Points at Springmoor

Lagerstroemia (Crape Myrtles) Continue to bloom, and give the Campus color.
Hibiscus syriacus (Rose of Sharon) is another small tree that blooms at this time of year.
Abelia grandiflora Kaleidescope (Kaleidescope abelia) Continue to bloom until frost.
Buddelia davidii (Butterfly Bush) Six foot tall shrubs with large flowering panicles add color to any garden at this time of year.

August things to do:

  • Remove all spent vegetables, and in mid to late August, plant collards, broccoli, cabbage, cauliflower, brussel sprouts, carrots, greens, and onion sets.
  • The above leaf crops are subject to get eaten by the white cabbage butterfly larvae. If you don’t like using Sevin Dust, then cover your leafy vegetables with bird netting.
  • Last application of 10 10 10 or 8 8 8 fertilizer to your ornamental trees, and shrubs. You don’t want to fertilize any further in the year, since new growth on plants will be prone to freeze damage later on.



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