We live in a high plains desert, where water is a problem. Denver only gets fifteen inches of precipitation in an average year, and much of this evaporates without infiltrating deeply, or comes as snow in the winter. On the other hand, in many years there is too much rain in the early spring, and summer thunderstorms can drop an inch of rain in a few hours, causing disastrous flash floods. In this post I will focus on how to cope with too little water; many of these strategies work both ways. I’m not going to discuss rainwater harvesting or greywater; both are very interesting and promising techniques, but are outside the scope of this post. There are lots of complicated details in regard to each; hopefully I will be able to give each of them a separate post in the near future.
Our focus is on using water wisely to grow annual vegetables and edible perennials, not ornamental gardens.
Also, I’m only giving a brief summary for each of the techniques below. As we implement them on the farms this summer, I will write a more in depth post on each with photos of our work.
Drip-line/ Soaker hose
These are a much more efficient way of using water then spray irrigation, because they don’t wet the leaves and soil surface. This also helps to prevent fungal disease. However, they are expensive, and tend to wear out over time. They are easy to damage with gardening tools. Also, they only work with clean, high pressure water, so they can’t accept rainwater or greywater.
Ollas/ Bucket Irrigation
Ollas are an ancient irrigation method. Small clay jugs or pots are buried near plants and filled with water. Depending on the moisture level in the soil, more or less water seeps out. Clay pots can be pricey. A cheaper alternative is a five gallon bucket with a small hole drilled in the bottom. Water will slowly dribble into the soil, soaking in deeply without wetting a large surface area. Buckets could be moved around to different beds, unlike ollas, which are immovable, but they lack the sensitivity to soil conditions gained with porous ollas. Delis and bakeries are good sources for free food grade buckets.
Wicking Beds and Containers
Wicking beds contain a subsurface reservoir of water, generally formed by a layer of gravel, which slowly wicks up through the soil to the plant roots. They eliminate surface evaporation and nutrient leaching, and keep the soil evenly moist, avoiding under and over watering. This is very important for some plants, such as tomatoes and lettuce. They are labor intensive and expensive to build. A cheaper variation is a wicking container built on the same principle. They can also be built from 5 gallon buckets.
This is the least tested of the ideas on this list. The basic principle of dew catchment is to insulate a smooth reflective surface, thus isolating it from ground heat at night. This lowers its temperature because of radiant cooling to the night sky. Once the temperature of the surface falls to the dew point, condensation collects and is funneled into a container or directly into the ground. Dew catchers can also improve the utilization of light rains, turning a surface dampening shower into a ground soaking drip of water on one spot. If you have any experience with dew catchment in Denver, let me know!
Organic mulch is a double edged sword from a water utilization standpoint. A thick layer of wood chips, leaves, straw, or other organic matter can retain moisture in the ground, and works well in combination with ollas, bucket irrigation, drip lines, and soaker irrigation. However, mulch can soak up a surprising amount of spray irrigation or rainfall before any reaches the ground. Since roots usually don’t grow in the mulch layer, this water is wicked away and evaporated into the air. However, when correctly applied, mulch can go a long way towards drought proofing a garden and has many other benefits.
Native Americans in the southwest used rocks as mulch in their gardens, which held moisture in the soil, and also increased the infiltration of light rains by shedding water rapidly into the soil. They may also capture moisture from warm air condensing on the cool lower rocks during the day. However, they can overheat plants and complicate the management of the garden. (For some plants, the extra heat is an added benefit.) In the wild, plants seem to grow lushly in and around talus piles at the base of cliffs.
Raising the organic matter percentage in the soil increases the amount of water that can be stored for dry times. A foot of rich soil can hold three inches of water. Also, deeply loosening soil can increase rooting depth and water infiltration. Double digging can achieve this on small sites, and chisel plowing on big ones. Hugelkultures, which are buried mounds of woody debris, achieve both these objectives. In this climate, sunken hguelkultures are probably better then the mounded types seen elsewhere. Mini hugelkultures can be dug into the ground for individual plants. Soil can also be contoured to catch water running down slopes and retain irrigation water, but this is a complex topic for another article.
Plant spacing can work both ways. A dense, Biointensive style planting can be lightly irrigated to create a moist microclimate under the leaves, slowing evaporation and speeding growth. However, wide spacing of large plants gives each plant access to more water, since the plant will react by growing a larger root system in the larger soil volume available per plant. In the end it depends on the objective. If one has a lot of room and hardly any water, then wide spacing is probably best. If one has only a little room and wants to use their available water to best effect, tight spacing will do well.
All else being equal, a plant in a fertile soil can get by with less water then one in an infertile soil. In a fertile soil with all the minerals in balance, a plant has to absorb and transpire less water to obtain its needed nutrients. This does not always apply, and fertilizer should not be overused. Also, woody plants should not be fertilized when they are water stressed.
How a plant is pruned makes a big difference in how much water it uses. This is a very complex topic, and I would advise you to do your own research.
A flat of transplants can be placed in the shade and watered more efficiently then the same seeds planted out in the eventual bed. This also has the effect of expanding the size of a small garden, since a bed can continue growing crops while the new transplants get going. On the other hand, some plants such as squash can sustain damage to the root system when they are transplanted, which reduces their ability to search for water. Large seeds should be pre sprouted until the root tip is just emerging, then planted. This helps conserve water and avoids root damage.
If plants can be started a few weeks earlier, when there is still abundant water available, things will be much easier later on. Cold frames, row covers and transplants are all valuable here. Also, deep waterings once a week are much better then shallow ones daily.
Some varieties are better at searching for water then others, and some types of vegetables are simply more drought resistant. For instance, purslane grows wild here in the summer with very little water, whereas lettuce is always thirsty and wilts in the heat. All else being equal, older varieties are more likely to be breed for tolerable performance in sub optimal conditions, but this is not always the case.
For plants that tolerate the shade, an over story crop or shade structure that blocks the wind and direct sun can make a huge difference in the amount of evaporation losses.
Soil that is too loose can keep water from moving upwards through the soil. If necessary, the ground should be firmed around new transplants and seeds.