As a designer I am constantly asked about installing a “low maintenance” landscape. For some reason I really get a kick out of this as people think that a living, growing landscape can look beautiful without any effort. Landscapes like people need care and nuturing for them to be good looking and successful. Water is vital to the survival of most living things. Plants are no exception. I would even dare to say that one of the main contributing factors to the death of a plant is water, either not enough or too much. It has been proven that plants that are irrigated properly (either with an irrigation system or manually) will grow at a faster rate and will have more health and vigor than plants that are left alone and let mother nature do the work. Bottom line; irrigation of plants is critical to the success and vigor of your landscape.
For trees and shrubs, once they are established (planted for a minimum of 6-12 months) require 1 ” of water per week. There are exceptions so know your plants! This may also be different depending on your soil conditions. Trees and shrubs in sand need more water as the water flows faster thru the soil. Plants in clay/loam soils which are typical here in Northern VA normally do best when they are getting an inch or so per week throughout the year, again this is a general statement. You can measure the amount of water with a rain gauge or cup placed in the area you are trying to track. If you have an irrigation system you can check it after you run your system or after there has been a rain event. Rain is tricky as amounts vary from different location only feet apart. I hear alot about “well it just rained so I do not need to water” when the fact is sure it rained for 10 minutes but you only had a trace amount actually getting to the soil and then due to the heat it evaporated back into the air before it got to the plant. This brings me to my next point: Plants absorb water thru their roots not their leaves! So when you are watering or an irrigation system is watering it is vitally important that the water is getting to the roots and not just hitting the foliage. Also any watering should be from the center of the plant out to at least the drip line (outermost foliage). Some plants for example; Japanese hollies have extremely dense root systems and water will not get to most of roots if you do not water them from the inside out.
Perennials, turf and annuals need more watering more often than most woody shrubs and trees. This is mainly due to the amount of root area and mass that can absorb water proportionally. For example you may notice when things get hot and dry that turf or other plants growing under a tree show drought stress sooner than others surrounding that area. The tree has a large well established root system in that area and has a tremendous capacity to literally suck that moisture out of the ground, where the smaller plants don’t have that much capacity, strength of root system, and even depth of their root systems to get to moisture that is in their immediate area. The success of any annual planting is generally due to constant watering until they are at least established. They don’t have the root mass, are planted in well drained soil media and are usually in raised beds that tend to dry out quicker. Turf also has a general shallow root system, especially in compacted clay soils, so they need to be watered 2-3 times per week, especially in the heat of the summer.
Here are a few critical watering things to keep in mind:
1) Water throughly. It is much better to water an area once per week for a long period of time vs many times per week for only a few minutes. There are exceptions: such as when plants have just been planted. This includes all plantings, especially sod, annuals and perennials!
2) Water the roots when possible. While usually it is better to water the plant any way you can vs. not watering at all, moisture on leaves in humid high stress conditions can lead to disease activity which can affect the health of the plant. Again if the choice is watering vs. getting water on the leaves by all means water.
3) Too much water in some cases can be just as bad as not enough. Boxwoods, rhododendrons, and many other plants don’t like too much or too little water and typically do fine if the soil is well drained. In the clay soils here in Northern VA, too much water that does not drain away can cause some root rot issues along with creating alot of the same stresses that not enough water creates. So know your soils and water requirements for your particular plants and adjust your watering accordingly.
4) Timing of watering. There is alot of misinformation about what time to water. The optimal time is typically early morning just before or as the sun is coming up, ideally everything is done by 10:00am. The thought is as the sun comes up it evaporates the moisture from the leaves that if left wet overnight in hot humid conditions can facilitate disease activity. The opposite can also be a concern; watering in the heat of the day between noon and 5:00pm. The thought here is that in the heat of the day alot of the water you are putting down is evaporating or can magnify the sun, thus burning the leaves (this is a rarity in my opinion and watering will cool the plant, although not as critical).
My suggestion is this: If you have an irrigation system run it in the early morning hours haveing the program end by 9:00 am or so. For example if it takes 4 hours for your system to run it’s cycle than start it @ 5:00am or earlier depending on the mowing schedule etc. If you can, do not water on the same day as it is mowed, just makes it easier on the mowing guys and the ground is firmer when not saturated. So if you are typically mowed on a Tuesday don’t run your system until Thurs. That give the mowers some flexibility in their schedule in case something happens in their routine.
As far as timing I am more concerned with getting the plants the water they need rather than not doing because of the time of day. With adequate moisture the plants are healthier and fend off streses better and will look better overall. Remember plants need water to survive and thrive, they don’t have the ability to just get up and go get a drink when they need it. It is up to us to help them when they need it.