The importance of the irrigation regime in the Dry garden: focus on sub-irrigation

The term Dry Garden refers to a garden with low water requirements. The term ‘low’ does not imply a total absence of water – which would obviously be impossible, because without water there is no life. Even low-water-use gardens may require irrigation. But it is a different process, carried out in different ways and only in case of emergency.

Until a few years ago, irrigating native plants and trees in a given area was not necessary, as woods and forests had lived for millions of years in the same climate, becoming resilient and able to survive without any external intervention. Today, unfortunately, this is no longer the case in many instances. Abrupt climate changes have caused plants in some Mediterranean environments to dry out due to now chronic drought.

We at BotanicalDryGarden experience this daily in our area. Here, in central Italy, there have been no prolonged rainy periods for at least 40 years. For water to penetrate deep into the ground, many days of light but continuous rain are necessary, something that happened in the autumn months several decades ago and that today seems impossible.

Both the planting of new gardens and the renovation of existing ones should include an irrigation system for convenience and, above all, to avoid risking the loss of the work already done. The focus we want to address is the amount of water to use and how to irrigate.

The initial stages after planting are the most delicate, especially if planting took place in spring. During this period, plants that have not yet established themselves well will have to face a hot and dry summer without being able to rely on an extensive and deep root system from which to draw nourishment and hydration. For this reason, the planting season makes all the difference, and if water is not available, the only alternative is autumn planting. In this case, a classic basin around the newly planted plants is the only tool that allows for substantial and precise emergency watering, which must be repeated whenever necessary. Naturally, the type of soil is also particularly important. Tough and generally clay soils require less water, while sandy and loose soils require much more attention.

Mulching also plays a very important strategic role. Covering the soil in the immediate vicinity of the plant with a layer of organic material (wood chips, bark, straw, hay, etc.) helps not only to control many weeds but also, and above all, to conserve water. Our projects also include mulching with inert materials such as pebbles, gravel, and similar materials, which are more suitable than organic ones for certain types of plants.

The most important technique for training plants to become self-sufficient is delayed watering. This consists in not watering shrubs and plants until they show the first signs of water deficiency, but waiting as long as possible and then providing a large amount of water in a single application.

This simulates abundant 20–30 mm rainfall, which encourages underground root growth by prompting roots to follow moisture deep into the soil. Obviously, this can only be put into practice if the soil is sufficiently permeable, which is generally achieved through very deep initial soil preparation.

On the other hand, frequent irrigation with small volumes of water encourages superficial root growth, and plants risk becoming easily stressed as soon as they go without water for just a few hours. For this reason, the use of drip systems must be carefully assessed and managed, always using the same irrigation technique: many hours, very infrequently.

Particular attention should be paid to irrigating lawns. Our alternative lawn varieties, which replace traditional English lawns, are ground-covering, treadable, hardy, and typically drought-resistant plants that require the same irrigation technique as shrubs: very abundant, very infrequently. Except during the initial phase after planting, which requires a few more millimeters of water more often.

With alternative lawns, there are not many options: you can choose between an overhead sprinkler system and a sub-surface drip irrigation system. Our experience increasingly leads us to choose the latter, especially for small areas and in the absence of ungulates.

The presence of wild boars and other wildlife, in fact, is not at all compatible with drip tape buried at a shallow depth. Latest-generation pipes, with root-intrusion-resistant internal drippers spaced 30–40 cm apart at a depth of 8–10 cm, ensure perfect surface coverage, allow for further water savings and, above all, make irrigated areas more comfortable to use, as there is no longer a need to move furniture and the spaces can be enjoyed even while irrigation is in progress. Furthermore, the absence of surface moisture also represents a major advantage in controlling the larvae of troublesome insects.

The only precaution recommended in this case concerns water quality: before deciding on this type of solution, it is useful to have the water tested to prevent any blockages that would make all the work useless. Once this has been ensured, however, sub-irrigation remains the most practical choice for low-water-use lawns.