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Watering needs change with the stages a plant reaches in its life cycle—particularly when getting started. Attention to these three phases—pre-germination, post-emergence and maintenance—gives the best chance to not only encourage and support vigorous plant growth, but to avoid disease and harmful fungus attack. It also makes for the most efficient use of water—important in controlling maintenance costs.
In all cases, and regardless of the growth phase, the overriding consideration in controlled water application is evaporation. Contributing factors include:

• temperature
• humidity
• wind velocity
• soil percolation
• slope angle
• sun aspect

Further complicating any adjustments for evaporation is sprinkler coverage. For this article we'll assume that the irrigation system is properly adjusted to give even coverage (see Water Audit).
Wind velocity and direction affect both evaporation and coverage and so must be considered carefully.

Phase I: Pre-Germination
To properly germinate, the seed must be kept moist at all times until just after the seed leaf is visible at the soil surface—a 3-day to 3-week process, depending on soil temperature and the seed itself. In high-evaporation conditions—full sun, steep slope, low humidity, high wind, etc.—watering may be needed as many as 8 times a day. Yet, overwatering must be avoided as well since long duration or high frequency can wash seed from the seedbed, leach fertilizer out of the root zone, disrupt the soil microbiology, lower the soil surface temperature and waste water.
For this first, critical phase, set irrigation system run and cycle times to keep the seedbed moist. This will vary with the depth the seeds are planted and whether a top dressing, such as Albright's Bed Spread, is used.
Turn the irrigation system on at midday under the average, expected wind conditions and measure the time it takes to uniformly wet the seedbed at the planned seed depth. Program the irrigation controller using this period as the system run time. Without further irrigation, check the seedbed every 15 minutes to see: 1) how long it takes for the soil to dry at seed depth, and, 2) which surface areas dry out first. Surface areas that dry out first are likely to be a result of poor sprinkler coverage—either misadjustment or misdirection caused by the wind. The period over which seed-depth drying occurs is the cycle time to set on the irrigation controller.
It is best to check seedbed irrigation coverage before planting to avoid disturbing seeds during testing or should work be needed on sprinkler coverage.

Phase II: Post-Emergence
Typically, as the seed leaves emerge and begin to shade the soil, evaporation will be mitigated, reducing the need for water. Another purpose in cutting back on moisture is to prevent disease. If humidity at the soil surface is too high, seedlings are vulnerable to a fungus infection called "damping off" that can kill new plants. Again, the balancing act comes into play since allowing the young roots to dry out will also be fatal to the new plants.
There are chemical treatments to control damping off but we are reluctant to use or recommend them, preferring a nontoxic biological control—a beneficial Trichoderma fungus marketed as PROMOT PLUS —that also stimulates growth and shortens the overall germination period.
Watering at this phase should be reduced to allow the soil surface to dry slightly before the next irrigation. A reevaluation of watering cycles and duration is necessary during this typically 3-week period that the young plants take to become established. The goal is to reduce the number of watering cycles while increasing the duration of each watering period—something like getting a newborn baby to give up 2 a.m. feedings. Depending on the plants, the season, the location and all the other factors to be considered, at the end of this period watering should be deep and twice a week.

Phase III: Maintenance
When the plants have become fully established, there is still a vulnerability to disease and fungus due to overwatering. At this point it is wise to employ evapotranspiration information to develop a watering scheme that can be calibrated and controlled using scientific data from the California Water Resources Board or the local weather service office. For a full explanation of evapotranspiration refer to Water Audit.

The Right Amount
Briefly, evapotranspiration rates express the amount of water used in growing vegetation. The ET number is based on the total amount of water that evaporates from the soil and transpires from the plant's leaves over a 24-hour period—the amount you will need to replace with irrigation. ET numbers vary with local geography. By knowing the total water needed to sustain plant growth then subtracting for evapotranspiration and adding for rainfall, watering rates can be fairly accurately adjusted to maintain optimum plant health and vigor.

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