Irrigation in ornamental plants
In a private garden, the way we supply water to the ground is essential for planning the necessary times and volumes. A general, very simple and intuitive rule is that we must not allow our blooms to reach the so-called “withering”, which consists of the condition in which the dehydration of the plant becomes irreversible. When the plant is dehydrate, the stage in which our plant presents a water shortage, we can act by watering.
Based on the size of the area we wish to dedicate to the flowerbed and the water needs of the flowers, shrubs and trees, we can decide whether to provide a real irrigation system, which will allow us to plan and schedule the frequency of interventions. Conversely, we will rely on manual method.
A plant in a pot or other container has its roots confined in a narrow and limited environment. Consequently, it will have different water needs than the same plant transplanted into the ground in a flowerbed. It must be watered more carefully, preferably when it shows signs of need. This can be observed when the soil has dried out sufficiently, more evidently when the leaves lose turgidity or even begin to wilt.
Irrigation needs vary from species to species: Some prefer to stay in drier soil, others in constantly moist soil, and still others can go for weeks without being watered and without wilting.
However, there is not a sure rule to water the plants in the same way throughout the year, but there are variables that determine the watering frequency and how much water to use.
The frequency, in fact, with which to water not only depends on the type of plant and its size, but also on the following factors:
- weather conditions: In winter the needs are reduced by 60-80% compared to summer.
- Temperature: The colder it is, the less the plants drink, so decrease the frequency and quantity of water.
- Ventilation: On windy days plants lose a lot of water through transpiration from their leaves, therefore they dry out more quickly.
- Air humidity: In humid weather and cloudy days transpiration decreases or stops and consequently also the absorption of water by the roots.
- Hours of light: During longer days, plants photosynthesize for more hours and therefore require more water.
- Sun exposure: The more hours the plants are exposed to direct sun, compared to hours of shade, the more the plants drink.
- Pot type and size: The terracotta pot is porous, so the soil dries out sooner than in the plastic pot and will require more water.
The larger the pot is in relation to the plant size, the greater the water reserve contained in the soil and the less frequent watering will be required. - Soil quality: High quality peat soils hold water better, like soils with some clay; vegetable compost or low-quality soil retain less water.
These factors vary in different specific growing situations; the quantity and frequency of water needed will consequently be different; the first rule therefore is to observe the plant often (every day in summer) to decide whether to water it or not.
In general, it is good practice to let the soil dry out a little before watering again, allowing the leaves to lose some of their turgidity. In fact, as the soil dries, it leaves room for air and oxygen, which is especially beneficial for the roots and the root hairs. If the soil is constantly wet, the root hairs deprived of oxygen will not be able to function well and the plant can suffer, even dying in the most serious cases by rot.
The root hairs, located at the tip of the roots, absorb the nutrients and moisture needed for the growth of the plant. They need oxygen to develop and perform their functions. To achieve optimal growth of the plant, the root hairs must be active. A slightly dry soil stimulates the activity of the root tips that develop by growing in all directions in search of moisture.
To test the humidity and to be able to establish if a plant needs to be watered:
- Stick a finger (a pencil) in the soil; it is thus possible to feel if it is dry or wet.
- Also see the color of the soil: if it’s dry it will be lighter.
- If the plant is in a plastic pot and the soil is dry, this will be very light.
- If it’s a terracotta pot, it will be enough to see the color of the walls of the pot to determine if a plant has been overwatered; in this case, in fact, the pot will be darker and slimy to the touch, sometimes it will form green algae.
If the problem is the excessive watering, the best thing to do is carefully examine the roots to determine their condition. The clod extracted from the pot must be solid and must not crumble, the roots must be white; if they are rotten, brownish and give off an acrid odor, it means that the plant is affected by root rot and it needs a specific treatment, if we are in time.
If a plant has undergone a period of water shortage and it has wilted, the best thing to do is to immerse the pot for 5-10 minutes in a basin of water at room temperature; in this way it will be able to quickly absorb a new reserve of water. If the pot is too large to be lifted, then do two waterings, not abundant, within 1 hour. Or maybe one abundant one if it has a saucer (removing any excess water from the saucer after two or three hours).
Attention: If the plant is wilted and the soil is wet, it means that there is too much water and you absolutely must not water until the soil dries out.
Transpiration, process through which plants absorb water, it is an activity that the plant carries out more when temperatures are higher and mainly during the day. Consequentially, it is good to water it in the morning, to ensure a sufficient water reserve to get through the hottest and brightest hours of the day, so that towards evening, the soil begins to dry and aerate to then be watered the following morning in the cooler hours. If it is very hot and a plant is very wilted already in the evening, it must be watered immediately to avoid yellow and dry leaves.
There is no single rule to water the plants correctly throughout the year, but there are variables that determine the time interval between two waterings and the quantity of water to use.
The frequency and doses of irrigation depend, on the one hand, on the soil’s ability to retain water, on the other hand on water requirement of plants which can vary a lot depending on both the species and the season. The highest water requirements occur where the soil is friabler and sandier, in full sun in the summer season and on fast-growing annual seasonal plants (petunias, begonias, impatiens, coleus, etc.). On the other hand, we have low water consumption on medium-textured soil where xerophytic plants (drought-resistant) are grown, such as many ornamental grasses (pennisetum, calamagrotis, carex, etc.), aromatic plants (rosemary, sage, thyme, lavender, etc.) or flowering plants such as Portulaca, Gaura, Euphorbia, Lantana, Gazania, Delosperma, Dipladenia, etc.
Flowerbeds with seasonal blooms averagely require more water, especially during the period of maximum flowering, compared to shrubs or grasses.
Irrigation methods do not only depend on the type of plant and soil, but also on the atmospheric conditions (temperature, wind, humidity), hours of daylight, the brightness and exposure of the flowerbed to direct sunlight.
After transplanting, in the first two weeks, while they are taking root in the flowerbed, it is important to remember that the flowering plants still have poorly developed roots and on hot days they easily wilt. Therefore, during this period they must be watered more frequently.
In general, when the plants are already grown, more developed, it is good practice to let the soil dry out before watering again, allowing the leaves to lose some of their turgidity and “call” for water. In fact, when the soil dries out, there is more free spaces in its porosity for air and oxygen, which is especially beneficial for the roots, which become stronger, stabilizing the plant.
At the same temperature, with high air RH, plants transpire less, therefore they absorb less water than on a day with dry weather. On windy and hot days, however, transpiration is maximum, therefore the soil and plants dry very quickly and it is necessary to water earlier or more.
The wind dries out the vegetation proportionally to its intensity. A light breeze is always good for plants, it removes excess humidity making them grow healthier and stronger. On the contrary, the absence of air movement for prolonged periods keeps the RH high and causes “mugginess” in summer, weakening the plants and making it easier for parasites to attack. Humid air in winter causes waterlogging that is harmful to leaves and flowers and promotes rots.
Irrigation needs vary from species to species: some prefer to stay in drier soil (grasses, succulents and xerophytes in general), others like and grow better in moist but drained soil, others still can go for weeks without being watered and without wilting.
The least demanding species for watercan be recognized because they almost always have small, narrow or more or less hairy/fluffy leaves or succulent ones with a shiny and waxy epidermis. Nature has structured them to limit leaf transpiration to a minimum and therefore the loss of water is conserved in the leaves and stems. These types of plants usually, if watered too much, suffer from excess water which can easily lead to root rot.
Between a flowerbed that is always in the sun and one that is always in the shade, with the same flower species, we can easily understand that the water requirements are very different, even reaching less than half in situations of strong shade. In exposure of partial shade but with sun from early afternoon until evening, the water requirements are very similar to those in full sun.
A good rule is to irrigate when the water temperature is similar to the air temperatureand therefore the temperature of the leaves, especially when we water directly on the leaves. If, however, the irrigation is under the foliage or underground, as in irrigation systems with drip hose, there is more freedom in choosing the time. In any case, it is always better to choose the morning, to ensure that during the hottest hours of summer days the water has already been absorbed by the plants, while in winter it is necessary to avoid the leaves remaining wet during the night.
For sprinkler irrigation systemsthat wet foliage and flowers, it is necessary to avoid watering during the hottest hours of the day, to avoid burning the petals (very sensitive) and leaves due to the lens effect of the sun’s rays on the water drops. In this case, watering should be done very early in the morning, ideally from dawn until 2 hours later.
In winter irrigation is very limitedand only in dry areas, where it does not rain or rains little, if there are flowering plants such as violets or cyclamens, you can water during the hottest days possibly in the central hours of the day. This also in the month of January, if it has not rained for a long time.
In most flowerbeds where there are herbaceous or shrubby perennial plants, in vegetative rest, irrigation is interrupted. Seasonal rains are sufficient.
In late spring early summer (T= 20-28°C) annual flowering plants grow very quickly and require more water, which must be provided to them at least before they begin to wilt conspicuously.
In summer, with very high temperatures and long days, watering must be frequent and their frequency depends even more on the hours of exposure to direct sunlight, on the species, whether it has high water needs or is resistant to drought, on the type of soil, whether it is sandy and draining or heavy (clayey) or whether it contains organic substances that help to retain humidity while maintaining good porosity for the roots and air.
Water just the right amount, according to the instructions given, but if there are still uncertainties the rule is simple: water a little less when the plants are large and have covered the ground with their foliage and flowers; they will tell you when they start to suffer, that is when the dehydration becomes evident and even before that when the leaves lose their turgidity. Water a little more when the plants are small and newly transplanted, but making sure that the species does not suffer from the cold. Cold and wet (in late spring) for sensitive species (Zinnia, Vinca, Celosia etc …) are a combination that can be harmful, even seriously, to the point of compromise taking root.