As a horticultural student at the National Botanic Gardens many moons ago, I was taught how to water. How hard, you might wonder, could that possibly be? Harder than you’d think is the answer, as shown by the countless garden plants that die each year as a result of underwatering, or overwatering, or watering so heavy-handedly that it damages delicate leaves and stems.
If, for example, you’re wondering why your up-until-recently thriving pots of seedlings have abruptly died without warning, the chances are high that their unhappy demise has almost certainly been caused by (a) overwatering, or by (b) overhead watering from a height, or (c) an unhappy combination of both.
The first – a good example of killing with kindness – greatly increases the chances of seedlings succumbing to the common fungal disease known as “damping-off”, by creating exactly the sort of cool, wet, airless growing conditions in which it flourishes. The second – overhead watering – can bruise and break seedlings’ fragile leaves and stems, as well as washing seed off the surface of the compost and down the sides of the pot or tray, never to be seen again.
What to do?
With seedlings, the first and most important rule of thumb is to water only when it’s needed, always allowing the compost to partially dry out between waterings.
Successfully gauging when that moment has arrived is a little like learning when a sponge cake is ready to be taken out of the oven, something that gets easier with experience. But there are always signs, once you start to look closely.
A good way to begin is by regularly examining the surface of the compost. Does it look like it’s becoming dry and powdery? Is it starting to shrink away slightly from the edges of the pot or tray? Do the seedlings themselves still look strong and healthy, or are they beginning to look just a little limp, their leaves a little dull?
Appearance aside, what does the compost feel like to the touch? Gently place your finger on it to judge (it should feel very slightly damp, but not wet and cold). Weight is another indication; if the tray or pot feels very light, then this is a reliable sign that it needs to be watered.
Putting aside the challenge of judging when to water young seedlings, consider the different ways to water them. One of the best is from below, by bottom-soaking the tray or pot up to its waist in a tray of tepid water. Allow the compost time to gently wick up moisture before removing it and then placing it in a bright, dry, protected spot to naturally drain.
I also use a small, hand-held pressure sprayer (the kind often used for misting indoor plants, with its head adjusted to give a fine, even, targeted spray) to very gently water young, fragile seedlings with barely-there root systems still finding their feet.
When it comes to watering garden plants, it’s a very different story. In this case, it’s very easy to wildly overestimate the amount of water you’ve given, as well as to underestimate quite how much water some plants will need at certain pressure points of the year. To put the latter in perspective, the RHS advises that during the growing season (spring to autumn), newly planted young trees or large shrubs can need up to 30-50 litres per square metre (four to six watering cans, or about a third of a small bathtub) each week during dry weather. Untreated rainwater is best, yet another good reason to rig up a simple rain-harvesting system.
Unseasonably warm, dry, windy spring weather, as experienced recently, is especially challenging for recently planted trees and shrubs, coming at a time when there’s already an awful lot of pressure on their immature root systems to deliver sufficient water to the rest of the plant. The same goes for any plants that have been recently moved or divided.
Again, how these are watered is crucial. A quick sprinkle is never going to cut it, especially when directed from a height and at a distance. Instead, direct the water very low down around the base of the plant and for long enough to properly penetrate the soil surface and reach its thirsty root system. Use a “rose”, adjustable spray head, or your index finger to soften the flow and prevent it from displacing soil, damaging any soft new growth, or exposing delicate roots to drying winds and bright sunshine. Spreading a mulch around the base of plants will also do a huge amount to slow down water loss and keep their roots cool, while simultaneously reducing water usage.
Here, overwatering is much less likely to be a problem. But bear in mind that popular watering aids such as sprinklers and drip irrigation are still best used sparingly, ideally only at those rare pressure points when a dangerous combination of drought, sustained high temperatures and lack of time (watering is very time-consuming) threaten new plantings. Use them too often and, the issue of water wastage aside, they’re also likely to cause problems with pests and disease, to damage flowers and blossom, and to adversely impact soil health.
As for keeping plants growing in tubs, pots, containers, hanging baskets and window boxes sufficiently watered, this is always challenging, especially during the growing season. Highly exposed to the drying effects of warm wind and sunshine, their root systems confined to smaller-than-ideal spaces, and typically grown in peat-based composts that are difficult to rehydrate once they dry out, plants often struggle.
In this case, useful tips include using as large a container as possible, avoiding those made of porous materials such as terracotta, and prioritising drought-resistant species that are naturally much less thirsty. Every once in a while, place the entire container (if liftable) in a wheelbarrow filled with a weak solution of liquid seaweed and leave it to soak. Placing a shallow saucer beneath the base of a container can also help by acting as a useful water reservoir. But just beware the inevitable downside, which is that in a wet summer, it’s often more of a hindrance than a help.
This week in the garden:
Keep potted-up dahlia tubers that are growing on under cover well-watered and take suitable precautions against slug and snail damage. Vulnerable to frost damage, dahlia plants in leaf shouldn’t be planted outdoors until late May at the earliest.
Earth up the young shoots of potato plants by piling up soil against them once they’ve reached a height of about 25cm, leaving just the upper 10cm above ground. This will encourage them to produce a good crop of their tasty tubers as well as protect the plants against late frosts.
Dates for your diary:
Irish Specialist Nursery Association show Farmleigh House, Phoenix Park, Dublin, Saturday, April 20th, 10am-6pm. The first show of 2025 with many specialist plant nurseries taking part. irishspecialistnurseries.com
Howth & Sutton Horticultural Society Spring Show and Plant Sale St Nessan’s Community School, Baldoyle, Dublin, Saturday, April 26th (2.30pm-4.30pm). With exhibits, plants and repurposed garden tools for sale. New exhibitors welcome. hshs.ie
Festival of Gardens and Nature Ballintubbert Gardens & House, Stradbally, Co Laois, Saturday, May 3rd & Sunday, May 4th. With a host of guest speakers including Nigel Slater, Fergus Garrett, Catherine FitzGerald, Andrea Brunsendorf, Rory O’Connell, Giuliana Furci, Amelia Raben, Charlie Harpur, Kitty Scully, Dominic West, Sally O’Halloran, Sean Pritchard, Sean Ronayne, Tom Stuart-Smith, Darina Allen, Diarmuid Gavin and Jamie Walton. festivalofgardensandnature.com
