Watering has been on every gardener’s mind all summer. Some have been dealing with water bans for months, while some have been fortunate enough to water as needed. I have made it a focus of all my newsletters this season, as it is the single most important aspect to the health of your living plants right now. I’m sure you can’t wait until you don’t have to pull out the flipping hose every couple of days.
Improper watering is at the core of most problems in people’s gardens and the drought just makes it harder. Over and under-watering by themselves are a problem, but they also contribute to insect infestations and disease. Some insects and disease happen anyway on many plants, but watering, or not, can trigger these problems too and take them out of balance.
Listed are seven major factors in determining how often and how much to water-
- Plants – Plants run the spectrum from high moisture needs( e.g., Rhododendron, Ferns, Astilbes, Macrophylla Hydrangeas) to plants that prefer dry soil and have low water needs (Lillac, Forsythia, Lavender, Sedum). It is critical to place plants with similar water needs in the same beds. Putting Macrophylla Hydrangeas in a bed with Lavender, Catmint and Echinacea will always be a challenge.
- Plant Size – A 10′ ornamental tree may need to get water 3′ feet deep into the soil, while a 1′ perennial may only need water to infiltrate 12″. Also, water that is 1′ deep will dry up much quicker than water that is 4′ deep in the soil. This is why lawns need regular watering and larger plants only require sporadic watering. You can water a perennial bed by hand with some time, but trees and large shrubs need to have a hose or soaker hose left at the base for several hours to infiltrate deep into the soil.
- Soil – Soil can be a challenge in New England with lots of rocky ledge, good organic loam, some clay and sand. The combination of these elements will form the ability of the soil to hold water. Clay, which is dense, has the highest ability to hold moisture, while sandy soil holds almost no water.
- Root Competition – This comes from congestion of plants, but mostly it comes from large trees and shrubs. In a drought, like we are having, trees, with their extensive root systems, will steal moisture. If you have a Norway Maple, that creates these dense carpets of fibrous roots within 15′ of tree, you understand the challenge of getting water through the roots to your plants.
- Sun Exposure – Gardens and lawn in the sun may need twice as much water as those in the shade, but that is also dependent upon the plant. Rhododendrons in the shade will need much more water over the season than Lilacs in the sun. Sunny gardens have to deal with higher temperatures and higher evaporation.
- Weather – Wind and temperature are major factors in evaporation and transpiration. Evaporation is moisture being removed from soil and grass, while transpiration is essentially plant sweating. The reason we like to water in the early morning is to minimize evaporation of water in the air and on the soil. Watering in the middle of a hot day can result in 50% of the water evaporating and never getting to the plant roots. Transpiration is most easily seen in Macrophylla Hydrangeas(blue/pink ones), as their big leaves cannot draw enough water to stay rigid on a hot summer day in the sun, but when temps and heat get in balance the leaves fill up again with water and regain their form. Their vascular system just doesn’t have the capacity to keep up with the transpiration on hot days in the sun.
- Natural rainfall – This is the variable this year that has caused so many problems. If you have an irrigation system and you just used the same setting as last year, you probably experienced some challenges in the garden and lawn. If you don’t have irrigation, then it has been one challenging and time-consuming effort in the garden to keep many plants alive.
Here is an interesting scientific article on water flow in plants from Nature. This discussion doesn’t even get into the more technical issues of nutrient impact, cation exchange, pH and other factors. You may notice sometimes if you fertilize your lawn in the heat, that it seems to take a step back and look weak. This happens because high fertilizer levels can interrupt the ability of grass and plants to take up moisture. This is why we try not to fertilize any plant material in real heat or drought conditions.
There have been people and studies that try to provide numbers that incorporate a lot of these factors into an algorithm to provide a numerical value to plants in specific situations. This gets to be too much in individual gardens, but is more effective for mono-cultures and crop production. Smart irrigation systems are getting better based upon local weather and conditions, but unless they are the systems sophisticated enough to incorporate multiple soil moisture sensors, wind and rain gauges, they can cause more problems and cause gardeners to be too reliant on technology. As we are finding this summer, one storm can deliver 2″ of rain in one town and 0.2″ in the next town over.
This is where the art of watering comes into play. A Rhododendron or Macrophylla Hydrangea can live in mostly sun or mostly shade. It may flower more in the sunnier spot, but it needs twice as much water.Typically a garden needs 1-1.5″ of water during the season, but during a 90+º stretch, it needs more, and if it is windy and dry, then even more. Often gardeners will let their plants tell them that they need water. Certain plants can be a bell-weather for a bed needing water, but that will eventually catch up and harm the plant if every week you wait for the plant to wilt. If you waited to drink fluid every day until you had a headache or stopped sweating, then your whole health would soon become compromised.
Lawns are less forgiving if you wait for them to start browning. At this point it may take weeks and excess amounts of water for a lawn to recover. You can let the lawn go dormant for the season, but with the complete lack of rain this summer, many lawns that were completely scorched may not return this fall. And with little fall rain, they will not have the chance to recover.
Moisture meters are an excellent tool and they range from this Rapidtest to a longer probed Lincoln. Care must be taken with the sensor tips as they can’t just be jammed into the ground. By going deep with a good sensor, you can tell the moisture level that you can’t tell by scratching at the top couple of inches of soil. The big problem with these sensors is that moist for a Rhododendron is different than moist for a Lilac. This is where overwatering becomes a problem. Gardeners will often look at the soil surface and think it is dry and add water, when in fact there is plenty of moisture deep in the root zone. Conversely, people may only be watering the top couple of inches while the plant is dry in the deeper root zone.
The single greatest problem in garden care of plants(not lawns) is under watering. Not necessarily how often plants are watered but how deeply. If you have a dry, open spot in your garden, water it for 30 seconds. Wait a minute or two and dig into the soil to see how deeply the water penetrated. Not very far, and if you have mulch, then barely into the soil. If you have a medium sized Azalea, that water needs to get 18″-2′ into the soil to effectively provide water to the roots. The primary reason that new plants fail to establish properly or die is that water does not get deep enough to provide for the deepest roots. Reed’s Rule: Water less often for longer periods. Well, it’s not my rule, but you know what I mean. Watering lawns every day or other day is more damaging and wasteful than watering once or twice a week.
So, if we take all of this information and create our plan for watering, we know that lawns, perennials and annuals are more shallow rooted and therefore will dry out more quickly, but we want the roots to go down for water, so we need to make sure the watering is going deeper than the roots. If we irrigate every day for a short period, the plants and lawn might live, but the roots will be very shallow and susceptible to a heat wave and to disease. These plants and the lawn only need to be watered twice a week, maybe one extra day in peak heat, but deep enough to get water 12″ down into the soil. Irrigation systems and sprinklers can handle this work easily. You can put a straight edged cup or bowl in the water zone and time how long it takes to fill it ¾”. Two waterings at that length of time a week will give you 1.5″. Bump it up 50% when you hit 90+º streaks in peak summer. This is for full-sun exposure. If the lawn and beds are in full-shade, you may only need 50-75% of the water depending upon the exposure. This is where overwatering often happens as watering from hot, sunny areas is replicated in shady spots. This will damage lawns and drown perennials and annuals that need some oxygen in the soil.
Now, shrubs need to be watered differently than these smaller plants and lawns. A good irrigation system should have completely separate zones for shrubs. Shrubs, without supplemental rain may need watering once a week, with less for well established plants, but they need to be watered for a much longer time to get the water much deeper in the soil. A moisture meter will really help understand what is happening 2′ down in the soil.
Established ornamental trees shouldn’t really need supplemental water during a normal season, but this year, a deep watering or two has been necessary. An established tree is one that has been in the ground for at least 3-4 years. Anything less than 3 years will need supplemental water with some regularity. Watering trees and large shrubs is the greatest challenge for gardeners as it takes several hours for a hose or soaker hose to get water 3′ down into the root zone. Soaker hoses or moving a hose several times around the root zone away from the trunk may take 3-4 hours hours minimum to properly water.
With all of these different types of plants, you need to focus on the plant species, plant size, soil, root competition, sun exposure, weather and natural rainfall to determine proper watering for your plants. The art of watering comes from being able to coalesce all this information and get into a rhythm for watering, paying attention to weather, and know the exposure differences from one bed to another. It is easy to over-water one part of your garden while under watering another part all in one week. If you are fortunate enough to have an irrigation system, spend the time to make sure each zone is delivering the right amount of water and learn how to easily manipulate it to deal with the different seasons and weather.
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