Assuring that the right amount of water gets to the roots of a seedling is essential in sustaining its life. One method to achieve this is called Deep Pot Irrigation. Deep Pot Irrigation includes an open-ended PVC pipe with holes drilled into it 2-inches to 3-inches down on the side of the pipe closest to the seedling. The pipe is then installed into a small hole dug next to the seedling. The bottom of the pipe should be no deeper than the length of the root plug. This assures that the water moves around the bottom and lower portions of the root plug where it can be reached by new, advancing roots. A screen is placed over the top of the pipe to keep animals out.
Pipe diameters range from 0.5-inches to 3-inches. Pipe lengths range from 8-inches to 18-inches.
The Deep Pot Irrigation method delivers water directly to the root zone of the seedling. So less water is required to douse and moisten the root.
The pipe used for Deep Pot Irrigation is filled with water periodically by hand or through a more sophisticated installed drip irrigation system. The amount of water delivered depends on the size of the pipe and how much water is poured into it. A seedling with a short root plug requires a shorter pipe while a seedling with a longer root plug needs a longer pipe.
A 2-inch diameter pipe holds four times the amount of water as a 1-inch diameter pipe, a 3-inch diameter pipe holds eight times the amount of water than the 1-inch diameter pipe assuming that the pipe length is the same. Determining the proper diameter pipe for your application of Deep Pot Irrigation is important. A pipe that is too big will release water beyond the root system of the seedling. Pipes that are too small will not fully moisten the area around the advancing root zone, requiring more frequent irrigation.
Pipe size and irrigation frequencies for Deep Pot Irrigation depend on:
· Soil Type. Sandy or rocky soils have a low water-holding capability. This causes the water to travel deeper into the soil and in a narrower band. So less water is necessary, but the watering will have to be done more frequently. The PVC pipe must be placed closer to the root plug of the seedling to assure that the moisture reaches the roots. Finer textured soils including loam and clay have a higher water holding capability and create a wider wetting area. In this case, more water can be applied, but at less frequency than sandy soils.
· Stock type. This refers to the seedling. Larger stock types have greater root volumes and greater aboveground vegetation to support than smaller stock types. So they will need more water. Smaller stock types may not need more water, but they do require more frequent watering.
· Seedling Quality. Healthy seedlings grow new roots quickly and can reach deeper soil moisture. Poor quality seedlings are slow to initiate new roots. So they must be watered more frequently.
· Competing Vegetation. Other vegetation growing in the same proximity as the seedlings will compete with them for the water causing in a depletion of moisture sooner. In this case, the seedlings will have to be watered more frequently.
· Species. Species of seedlings have unique rooting patterns, growth rates, and water needs. Species that grow roots quickly and have a higher rate of water withdrawal require larger, deeper pipes and more frequent watering. For more information confer with a nursery manager concerning the species in question.
Deep Pot Irrigation not only provides direct watering of seedlings, it also permits the introduction of soluble fertilizers and mycorrhizal fungi inoculum. Treatment of seedlings in this manner helps to limit weeds that may appear on the soil surface that can compete with the seedlings for water.
Take care when determining fertilizer rates. Improper rates of fertilizer can cause the increase of soluble salts that could be toxic for root growth. To assure that salt levels are tolerable, measure and monitor the pH and salt levels of the water.
Finally, moisture stress status of the plant can also help to determine when to water. One method of determining plant moisture stress (PMS) is to use a pressure chamber, which measures plant stress in negative bars at the time of the reading. PMS readings should be performed in the early morning before sunrise. it is this time that PMS should be at its lowest. If the pre-dawn PMS reading is less than -15 bars, the seedlings are under high moisture stress and should be watered as soon as possible to keep the seedlings from dying.
If the purpose of Deep Pot Irrigation is for fast seedling growth, then PMS during the growing seasons of spring and fall should be maintained at above -5 bars.
Although plant moisture stress equipment is expensive to purchase, Forest Service district offices use them.
(Photo and illustration courtesy of nativerevegetation.org)