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Watering scientifically

Did you know that proper watering is a lot more complicated than providing your garden with that critical one inch of water per week?  To figure out the best watering method for your area, first figure out how much water needs to be lost before vegetables are stressed using the chart below:

Chart 1

Soil type
Amount of water lost before vegetables experience moisture stress (inches of water per foot of soil)
Sandy
0.5 to 0.75
Medium
1
Clayey
1.5

Next, pick out your climate zone from this chart:

Chart 2
Climate zone
Inches of soil moisture lost per sunny day in the summer
Cool (western Washington)
0.2
Moderate (northern U.S.)
0.25
Hot and humid (mid-Atlantic and southeastern U.S.)
0.3
Hot and dry (prairies and northern California)
0.35
Low desert (southwestern U.S. and California)
0.45

Then plug your numbers into this formula:

Days between watering = Chart 1 value ÷ Chart 2 value

For example, in our clayey soil in the hot and humid climate zone:

Days between watering = 1.5 ÷ 0.3 = 5 days

So, every fifth day, I need to add 1.5 inches of water back to the soil.  My father, who lives in the hot and humid zone too but has sandy soil, may need to water every day or two but will add less water each time.

Of course, to maintain a perfect watering schedule, you need to keep track of weather conditions.  On cloudy days, the soil doesn't lose much water, so you can wait an extra day to water.  When it rains, the amount of precipitation can be added back into the soil's supply, putting off watering even longer.

Impulse sprinklerAs further evidence that Steve Solomon and I are on the same wavelength, Gardening When It Counts gives the same irrigation advice I do --- buy high quality impulse sprinklers and ditch the trendy drip irrigation.  Solomon notes that drip irrigation equipment is expensive, short-lived, troublesome, easy to cut through, and prone to shifting away from the plants it's meant to water; and doesn't work in sandy soil because the water won't spread horizontally; but is potentially good for permanent plantings like raspberries.

For other prime gardening advice --- like why you should never water seedlings --- go read the book!

Want to know how we make a living on the land?  Read our microbusiness ebook.


This post is part of our Gardening When It Counts lunchtime series.  Read all of the entries:




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Thanks for this, the formula is so useful! I need to check out that Solomon book too -- most people seem to advocate drip irrigation so strongly.
Comment by Eliza Thu May 6 14:24:23 2010
I'm just guessing on the watering seedlings. Does it cause a rot around the "neck" of the plant? I've really been struggling with starting herbs. They get too dried out during the day, then watering from below leads to sogginess and watering from above leads to drowning : ) Any other input from folks out there?
Comment by Erin Thu May 6 14:46:32 2010

Water seedlings in the morning.

Watering? A garden? Are you kidding?

Seriously, After three weeks of drought, my strawberries, mulched with newspapers and some wood chips to hold them down, had moist soil around their roots.

Comment by Errol Thu May 6 15:35:39 2010
I have a 300gal fiberglass tank buried in the ground with washed gravel poured around the tank to catch rain water run off. The plan is to pump water from this tank to a 2000gal tank on a tower. If I use gravity feed I think soaker hose would work best for garden plants and drip irrigation for trees and shrubs. There just wouldn't be enough pressure to run sprinklers.
Comment by zimmy Thu May 6 16:20:45 2010

Eliza --- that's what I like most about the book; it isn't afraid to take the unpopular stance. We haven't had much luck with drip irrigation here, but that has a lot to do with our water source (the creek).

Erin --- I'm glad you bit. :-) I wanted to talk about his ideas on seedling watering, but the post was already way too long and I didn't want to send too many people scurrying for cover. Solomon made the very good point that when you water a plant, you decrease the soil temperature drastically. As Daddy points out in a later comment, watering in the morning (well, late morning) helps with that, but seedlings lose a lot of vigor when the soil temperature drops. Solomon's solution is to use those seed-starting trays --- wet the soil down, put on the lid and let the soil warm back up, then plant your seeds. If you keep the lid on until the seedlings are up and growing, you won't have to water, so the soil temperature will stay high and prevent the formation of fungi that cause damping off.

Daddy --- we've noticed that yields go way up when we water. On the other hand, you use a lot of Solomon's spacing suggestions, which means that your plants have room to range far and wide for water. On the other other hand, Solomon makes the point that the damp soil right under a mulch doesn't mean that there's actually enough water in the soil for your plants to "drink."

Zimmy --- you're totally right about pressure. We didn't really get our sprinklers going until we got a massive pump (free with the thousand gallon tank we got so cheap on Craigslist!) When we were watering on a low pressure system like yours, it was extremely time-consuming and painful. If you have any sediment in your water, it will clog up the holes of your soaker hose and you'll end up with big dry spots and spend a lot of your time running around with a pin cleaning the holes out. Our little yellow stationary sprinklers worked much better in that situation, but you do need a lot of them to water a large area.

Comment by anna Thu May 6 16:44:36 2010

HUGELCULTURE INTRO Check out "Hugelculture", or hugelkulture, a German word relating to mound culture.

In brief, at the base of a raised-bed-to-be, put logs and large sticks, or wood chips, or lumber, all of which can be rotting or rotted highly. Why?

To make a catchbasin for nutrients and water. To form a mini-ecosystem under the soil where micro-organisms can have a place to call their own. So that you won't have to water very much at all, and the are will be largely self-supporting for a good many years. This seems better than either Steve's system, and drips, at least for many vegetables, perhaps not greens.

LINKS FOR HUGELCULTURE

This is a simply great thread of discussions on HC. It is based on the system popularized by Sepp Holzer, the man who in the Austrian Alps grows citrus and all manner of amazing things. He has been referred to by at least one man as a “permaculture god.” ☺ http://www.permies.com/permaculture-forums/17_0/permaculture/hugelkultur

TUTORIAL http://gaiacraft.squarespace.com/hugelkultur-lesson/

VIDEOS http://www.youtube.com/watch?v=PnfiWm_O3o4

http://www.youtube.com/watch?v=62ge_AqaVtM

Comment by MMC Wed Apr 27 21:21:52 2011
We've actually done quite a bit of experimenting with hugelkultur already. We use it in our forest garden for our perennials right now, but someday we might add it to our vegetable garden. To see the methods we've used, just type "hugelkultur" into the search box on the sidebar. I agree that hugelkultur has a lot of long term potential, but it's not going to be the short term solution for a vegetable garden in the next year or two.
Comment by anna Wed Apr 27 21:28:19 2011

One very unique homestead, $1,500 per acre, the opportunity of a lifetime