Not-so basic watering

How much and when. This is the most common question I’m asked about gardening. The expectation generally is that there is an answer. And that the answer is something like: 7.

But watering is very complicated if the goal is to provide every plant with the optimum amount of water, just at the point in the plant’s life when it is needed. Add in the consideration of the soil type; presence of mulch, surrounding trees, buildings or fences; the amount of sun and wind exposure… and quickly the question becomes unanswerable. So I want to try to give advice on watering based on the goal aimed at by the gardener:

  • Have all the plants do pretty well while following a basic water rule: My rule would be to water deeply once or twice a week between late May and September. During heat waves, every 3 days. A deep watering is 2-3 hours (depending on water pressure). Do this whether it has rained or not. Most rainfalls are not enough to sustain gardens and lawns in a densely-planted urban forest. I see gardens struggling with too little water fairly frequently; I rarely see an over-watered garden.

This basic regime can be infinitely fine-tuned to respond to different needs and conditions.

Some of the special conditions you might want to consider are:

  • Getting newly planted shrubs, trees and perennials established: Water the plant thoroughly in its pot before planting. Experienced gardeners often submerge the pot in a pail of water to ensure the root ball is completely saturated.  When planted, water the newcomer immediately. In general, plants put in the ground during active growing periods need about two weeks of regular watering, every 2 or 3 days. More often if it gets very hot and/or windy. Shrubs and trees will need regular watering for the first 2 years while they build more extensive root systems. All newly planted shrubs, trees, and perennials need extra water during heat waves. A lack of water is the number one reason why new plants fail to get established. If new trees or shrubs are under water-stress in the first year, this can stunt their growth for years to come or cause them to die if any new stress occurs. In general, the larger the plant, the longer it will take to build a root system adequate for its needs, requiring a longer period of special care. When newly planted, the entire root system of a plant is confined to the zone immediately beneath the foliage. During a rainfall, the foliage will deflect the water to the drip line or beyond, out of reach of the roots. So unless there is a very heavy rain, the roots will not get adequate water even if the soil in the garden looks moist. The roots are also often in a soil (used by the nursery) that is different from that of the garden at large. Water can run down the gap between the two soils instead of moistening the soil around the roots. Breaking up the soil root-ball to some extent, will help alleviate this problem. The roots of a potted plant are still confined to a restricted area when newly planted, so this area requires extra water until the plant has spread (grown) its roots more widely to access greater water resources.
  • Gardening with large trees nearby: This is a typical situation in Toronto and needs special consideration. Many gardeners, and I’m one, have an ambivalent relationship with trees that share our gardens, generally trees not chosen with our gardens in mind. Often these older city trees are non-natives, such as the shallow rooted Norway Maples. Trees will out-compete smaller plants when it comes to scarce water resources. Trees have very wide root zones, reaching far beyond their drip-lines. And trees need a lot of water from when they start to leaf out until they start to lose their leaves in fall. They will grow roots into any area that receives water, whether from rain, a downspout, or watering by a gardener. If you only water the areas of the garden that have shrub or perennial beds, trees will expand their roots in that area to benefit from the available water. Not watering will put both the trees and the other plants in the garden under stress. Trees under stress can drop large limbs as a desperate measure to conserve resources during drought. The solution: water the entire garden to provide adequate moisture for the trees and other plants. Trees in the city can not be self-sufficient because their root zones have been extensively disrupted and restricted by repeated building and paving over much of the available ground. Water that falls on paving is largely diverted into the storm sewers and hence is not available to the trees. The original soil in Southern Ontario, before any extensive building, was 2 feet deep and full of water-retentive organic matter from thousands of years of soil-building activity in the native forests. Today in many gardens, the fertile soil layer dumped in place after construction has removed or buried all the original soil, in repeated cycles of building and demolition, is barely 4 inches deep. To assess the water requirements of trees in or near your garden, compare the planted or open soil areas to the paved or built-upon areas. If half the ground is paved, the water need is correspondingly greater. Even where this is less of a problem, the root zones of trees should be given regular water during droughts and heat waves. Again, the ideal is to water deeply every week to encourage the development of roots in deeper layers of soil, rather than at the surface. Roots will grow where there is water available: as the soil dries between waterings, the deeper layers stay moist longest and thus roots will grow there, making trees and perennials more drought-resistant.
  • Caring for evergreens: Unlike deciduous plants, evergreens need extra water in the fall and early winter. Because they continue to lose water from their tissues all winter, they need to store up water before the ground freezes. Even if the soil in the garden seems moist in the fall, often the soil immediately around and under the evergreen will be dry because the roots are absorbing all the available water. A good rule of thumb is to water evergreens, especially if they are large, about once a week in October, November and December. Give them each one or two large watering cans of water. Newly planted evergreens, in their first or second seasons, might need even more. Wind dries evergreens very quickly, even if temperatures are cool and the ground generally seems moist. Either test right under the evergreens for soil moisture, or simply water them regularly. Toronto has experienced several dry springs and low snow/melt-water levels in the last few years and evergreens have also suffered drought in spring and should be given extra water.
  • Soil type: Sandy soils will require more frequent watering than heavy clay soils. Sandy and clay refer to the mineral content of the soil. Soil is made up of a mineral substrate and organic matter. Sand has a large particle size with corresponding large air pockets. Clay is made of very small mineral particles, tightly packed, and tends to have very little air available. Roots require both water and air in the soil. Any soil that is completely water-logged will have no air trapped and the roots of plants tend to suffocate. When water is added to sandy soil, it tends to trickle down quickly and penetrate deeply, but correspondingly is quickly lost from the root zones of plants. Clay soils will tend to repel water at the surface when they are dry, because they get hard-baked in the sun. They may also have fissures when dry, down which water can vanish before it is absorbed by the root zone. Both problems are mitigated by watering slowly over a longer time. Weeping hoses are better for this than sprinkler systems because they release water more slowly. Clay soils tend to remain moist longer because of their denser structure, but will also require more time to absorb water. It is surprising how much rain is required to thoroughly moisten the root zones of plants in a heavy soil. Rain of 10 mm or more is necessary to be effective for plants. Test the water content of the soil with a trowel, digging down a couple of inches: moist soil will be a darker colour. Touch it to see how moist it is and to learn to relate the colour to the moisture content. Your soil is specific to your garden, determined by its history, especially the building history, and how effectively it has been gardened. Often contractors will replace the top soil lost during construction of any kind, with only 3″-4″ of top soil and install sod on top. The shallow soil will quickly dry out and require frequent watering of the lawn. If you replace such a lawn with a garden bed, you will need to improve the soil with organic matter to create a healthy dark soil about a foot deep. The deeper the soil, the deeper the roots will grown and the more water will be available to plants, requiring less watering. The earth left behind after construction is generally sub-soil, of a pale colour and uniform texture (clay or sand or a mixture). Organic matter in the soil will make it look darker. Whatever the specific composition of the earth in which you want to garden, it can be improved with the addition of more organic matter. There are only a few plants that prefer soil with low organic content. The organic matter will soak up water like a sponge and release it gradually to the plant roots. So the frequency of watering will depend on the amount of organic matter as well as the mineral (clay/sand) composition.
  • Using mulch: Mulch is anything that covers the soil, slowing evaporation and lowering soil temperatures. Common mulches are shredded bark (mostly cedar and pine), chunks of bark, and decorative stone (gravel, pea gravel, river rocks). Organic mulches should be 3″ to 4″ deep. They have the added benefit of enriching soil micro-organism, fungal, and invertibrate communities. These in turn make nutrients available to plants. So over time, the organic mulches are broken down and turn into fertile soil, requiring re-application of the mulch, usually every season. When your mulch is disappearing, this is a good sign of a healthy garden soil. There are some plants that don’t like organic mulches: these are generally the Mediterranean or drought-tolerant plants that thrive in a sun-exposed gravel garden. For these plants, coarse sand, river rock or gravel mulch is best. A garden that is mulched will require less frequent watering. To check on soil water content, move aside the mulch and use a trowel to see if there is still available moisture an inch below the soil level. Be careful not to let mulch touch the stems or trunks of the plants. Keep a 6″ circle around the crown of perennials clear and a foot around shrubs or trees.
  • Shade gardening: Many shade-tolerant plants also prefer a more moist soil than sun-lovers. It is helpful to group these together to make it easier to give them extra water. This makes aesthetic sense too, since we tend to experience plant associations that one might find in nature as more harmonious. A 3″ deep layer of mulch around these plants will help the soil stay moist longer between waterings. If using a weeping hose, loop it more densely across the soil around moisture lovers; this will ensure they get more water than those plants in areas where the hose runs straight.
  • Growing sun-loving plants and drought-tolerant plants: They often dislike having wet leaves and can benefit from being watered by a weeping hose rather than an overhead sprinkler. They are generally fine with one thorough watering per week, except on sandy soil when they may need more.
  • Planting near a wall or fence: The ground within a foot or two of a wall or fence may be in a rain-shadow, depending on prevailing winds. It is best not to put plants in this zone, especially those that require regular watering. In small gardens, enclosed on several sides, this one or two foot edge amounts to a significant part of the garden and other solutions for ensuring sufficient water for the plants must be found. Even when the rest of the garden looks well-watered by the available rainfall, in the spring and fall, for instance, the plants in the rain-shadow of structures can be experiencing drought and require water. This effect can be a benefit for spring bulbs: planted close to structures, they thrive on the dry-summer conditions if they get some water in the fall to start root growth and in spring to support active growth.

While this looks very complicated, you will quickly develop a routine for responding to the water requirements of your own garden. When making plans to change your outdoor space, consider the impact these changes may have on the living infrastructure of your surroundings: the plants and trees that live there with you and need water and healthy soils to thrive.