After planting, it is important to make the your leylandii trees get enough water and that the roots never dry out.
If you plant between November and February, a your Leylandii trees will need less water (or maybe even none at all). However, you can plant container-grown (pot-grown) Leylandii from March to October without a problem as long as you water them throughout the spring, summer and autumn (but not in the winter). Keep an eye on them from February to October to make sure the roots don't dry out. After they have been growing for a year, they generally do not need watering again as their roots will grow to take up the water they need.
How to water your Leylandii
Firstly, check they need watering by pushing your finger into the rootball of the plant or into the soil next to the rootball. This should be done every 2-3 days for the first year (spring, summer and autumn). The soil wants to stay moist but not soggy or waterlogged.
In general, your new Leylandii trees will need a good soak once or twice a week, every week (maybe more in very hot and dry weather) but this will depend on the weather conditions and your soil type. If you go away for a couple of weeks during the summer, make sure you have an automatic irrigation system and/or someone reliable to water them while you are away. If not, you could come back a find them all dry and crispy!
You can water using a hose or watering can but make sure you water the soil around the rootball until the water starts to run off, then move on to the next plant and work your way along the hedge. To make sure the water has sunk in, you should go back to each plant 3 to 4 times - give the water a chance to sink in to the soil before watering each plant again.
Alternatively you can use an irrigation system such as Porous Pipe (leaky hose). This is good as it allows the water to sink into the soil at a gradual rate. As a general rule of thumb, water them once a week for about 2-3 hours with Porous Pipe - up this to two or three times a week during very hot and dry weather. Remember to check that they need it first as it is easy to overwater Leylandii with irrigation systems.
Never assume that just because it has rained that your Leylandii have had enough water. Often rain in the spring, summer and autumn months is not enough to provide enough water to the roots. If you buy a rain gauge, you will know how much water they are getting from the rain. As a rule of thumb, Leylandii need about ¼ inch (6mm) of rain to keep them watered for a 2-3 days during the summer months. If you are getting less rain than that, you will need to test the soil to see if they are drying out.
Don't leave it until your Leylandii look like they are wilting or suffering from drought before you water them.
Watch out for drought symptoms
When Leylandii are too dry or too wet, their needles start to turn yellow and then brown. This starts at the base of the plant, near the trunk and works its way upwards and outwards.
The symptoms of drought are normally caused by lack of water but can also be caused by too much water (waterlogging). This often occurs when people leave irrigation systems on too long or when Leylandii are planted in small holes in heavy clay. If you have a heavy clay soil, make sure you break up the soil when planting Leylandii so any excess water can drain away, otherwise you can create a clay bowl for the water to collect in.
The roots of Leylandii will rot when they sit in waterlogged conditions for extended periods of time. This will mean your Leylandii cannot get enough water to the foliage and they will look like they are suffering from drought when they are actually too wet.
As soon as you have planted your Leylandii trees, you can start trimming any branches that come out wider or that are taller than you want the hedge to be. If you imagine the shape you want your hedge, trim off any branches that protrude out of this shape. This will encourage shoots to grow within the hedge and will ensure the hedge thickens out quickly.
When the height of your Leylandii trees grow to within 6 inches (15cm) of the height you want your hedge to be, trim off the tops. This will also help the plants thicken out.
Twice a year, or more often when the hedge is establishing, trim off any shoots coming out of the hedge or growing taller than you want the hedge to be. The hedge will start to thicken up and form a good screen.
Once the hedge has established, you need only trim once a year.