Planting a Leylandii hedge should be a relatively easy job. It is a very simple exercise but it is very important to get it right for the quick and successful establishment of the hedge. If you make it easy for your Leylandii plants to get a good root system established quickly, then the hedge will form quickly.
Clear the area to be planted of weeds and/or grass. You can spray a 2-3ft (60-90cm) strip with a glyphosate-based weedkiller (such as Roundup, Tumbleweed, Bayer Glyphosate or Resolva Total Clear). As soon as the weedkiller has dried, you can plant the Leylandii as glyphosate has no residual effect. This is normally 3-6 hours.
Dig a square hole about twice the width of the rootball of the plant. Sometimes it is easier to dig a trench, depending on how far apart you are planting the trees.
Dig the hole about two inches (5cm) deeper than the rootball of the plant as the soil level will drop over time and you do not want the top of the rootball protruding out of the soil.
Break up the soil in the bottom of the hole for at least another 6 inches (15cm) - deeper if you can - this will allow the roots to get into the soil more easily and also allow any excess rain or water to drain away.
If the soil is good, you only need add some 12-month controlled-release feed (such as Multicote or Osmocote) to the soil to give your plants all they need for the first year. After that, if you have planted them properly, they should get all the feed they need from the soil. We recommend 50g of Controlled-Release Feed per plant for plants up to 4ft (1.2m) tall and 100g per plant for plants over 4ft (1.2m) tall. Mix the feed with the soil in the bottom of the hole.
If the soil is poor or you want to get your plants off to a quick and easy start, then you can add compost to the soil. The quantity will depend on how poor the soil is but if it is fairly reasonable, add approximately the same amount of compost as the size of the pot they are in, for example, if they are in a 10 litre pot, then add 10 litres of compost per plant. We recommend a John Innes enriched multipurpose compost. Obviously, if you are putting compost in the hole, you will need to dig out more of the soil so the rootball is not proud of the soil when planted.
If the soil is poor or conditions are difficult in any way, then add Rootgrow Mycorrhizal Fungi to the soil. This is recommended by the Royal Horticultural Society (RHS) and helps the plants develop a massive secondary root system within a few weeks. This helps the plants establish quicker so you get quicker growth and also more drought tolerance. Try and get as much of the Rootgrow on the roots of the plant and in the bottom of the hole exactly where the rootball of the plant will sit. Mixing Rootgrow in the compost is not effective, it needs to be touching the roots to survive.
Place your Leylandii in the hole and back fill with soil and/or a mixture of soil and compost. Firm the soil around the rootball with you foot. Make sure the top of the rootball is about 2 inches (5cm) below the surface of the soil.
Stake the plant if it is over 4ft (1.2m) unless it is in a very sheltered location. A stake will hold the plant from rocking in the wind. If the plant rocks in the wind, the newly established roots can break and this may make it difficult for the plant to survive. We recommend staking your plants about 12 to 18 inches (30-45cm) from the ground with a tree stake driven in to the ground at an angle of 45 degrees. Angle the stake so the end is pointing into the prevailing wind. This way, a strong wind will blow the stake into the ground rather than out of it! Tie the stake to the tree with a tree tie or an old pair of tights. Cable ties are not recommended as they can damage the plant. The stakes can be removed after 12 months.
If you are using a digger to dig a trench, make sure you break up the bottom of the trench so the soil in the bottom of the trench is not "smoothed-over" by the bucket. This will allow any excess water to drain from the trench and will also allow the roots to get into the soil.
If you are going to re-plant a Leylandii hedge where one has been growing previously, it is best to dig out the old stumps. This is easiest with a digger if you can get one into the area. The soil around an existing Leylandii hedge will be depleted of nutrients and matted with roots so it is best to dig a trench 2ft (60cm) wide by 2ft (60cm) deep and replace all the soil with good top soil. If you do not do this, your new hedge will struggle to establish as even Leylandii do not grow well where an established Leylandii hedge has been growing.