While I was watering the other day, I was wondering how long to keep the hose on each bed. Of course that depends on many factors, but I’m looking for a rule of thumb.
First I need to know how much water comes out of the hose:
With the water on full and the nozzle on “shower”, it took almost exactly 1 1/2 minutes to fill a 5-gallon bucket. So that’s 3 1/3 gallons per minute (200 gph).
Our garden beds are about 6×9 feet, or 54 sf = 7,776 sq in. So 1 inch of water on the whole bed is 7,776 cubic inches. There are 231 cubic inches in one gallon of water. So it takes 33.6 gallons (evenly spread) to get one inch of water onto the bed. At 3.33 gallons/minute, that’s about 10 minutes.
How much “rain” does a vegetable garden need? According to numerous sources (like this one), about one inch per week, total. That’s convenient – 10 minutes of hose time per bed per week, evenly spread, if it never rains.
Of course, that’s a very broad guideline. Mature plants with deep roots could probably be watered with the whole inch once or twice per week. Newly planted seeds, seedlings, and transplants need to get water probably every day, because the roots are so shallow and the little plants have no capacity to store moisture.
Too much watering can be almost as bad as not enough. Soil around mature plant roots needs to dry out so the roots can get oxygen. Sometimes yellow leaves indicate too much water. A number of sources say that watering tomatoes too much compromises the flavor (like the discussion in this article about “dry farming”, which claims that stressed tomatoes taste better).
Wilting in the hot sun is a natural reaction by the plants to slow transpiration, so doesn’t necessarily mean that the plant is water-starved. Dig a little hole and see if the soil is really dry before getting worried.
So the answer to my question obviously depends on what I’m watering (mature plants with deep roots – or not), when the bed was watered or rained on last, and the forecast for rain or watering. Therefore my rule of thumb will be never more than 10 minutes on any one bed, usually probably only a few minutes – especially if the soil is already moist a few inches down – and maybe slack off a bit on the tomatoes.