Famous for its use in hedges and windbreaks, the cedar is a splendid evergreen tree that can also be installed as an isolated subject in a garden. Let's see how to grow it in the best conditions to avoid the diseases and parasites that sometimes attack it.
How to water the cedar ?
Watering should be regular and abundant for the first 3 years after planting to ensure a good recovery. However, the soil must not remain constantly wet! It is all a question of dosage.
Automatic drip watering at the foot of each tree is recommended for hedges throughout the summer.
In winter, water every 8 days if it does not rain.
Beware, in Mediterranean regions: the winter drought can cause water shock in the first few years after planting, so be careful!
How to plant a cedar ?
Never plant a cedar in a constantly wet soil, it will not resist for long. Clay soils that remain very wet induce the appearance of the dreaded Phytophthora and other cryptogamic diseases. A plantation on mound with addition of draining material could be considered in these conditions.
- Dig a hole twice the size of the root ball in all directions so that the cedar can develop its roots well.
- If the cedar is purchased 'bare root' in the fall, prune the roots before planting.
- If it is potted, soak the root ball in a basin of room temperature water to hydrate it, then carefully remove it before untangling the roots.
- Pour a good layer of draining material at the bottom of the hole (pozzolan, gravel, stones...) to ensure good drainage.
- Spread a mixture of garden soil, river sand and compost on top.
- Add mycorrhizae powder to ensure good vigor and better resistance to pests and diseases to your tree.
- Adjust the height of the soil at the bottom of the hole so that the collar is flush with the soil surface.
- Spread the roots at the bottom of the hole.
- Backfill, packing the soil as you go.
- Form a basin around the foot to retain water.
- Water abundantly to evacuate the air bubbles present in the soil.
How to prune cedar ?
Sharpen and disinfect the blades of the pruning tools with methylated spirits or a flame before pruning. Choose hand shears or thermal or electric hedge trimmers (depending on the size of the hedge and its location in the garden). The blades of these tools should be kept parallel to the hedge to ensure effective trimming.
Wear protective clothing before pruning.
Remember to water the hedge well the days before pruning.
Maintenance pruning of single trees:
- Cut the dead wood short in the fall with a saw.
- Prune damaged branches with secateurs.
Pruning cedars in hedges:
- Prune twice a year in spring and fall. For aesthetic reasons, it is possible to prune during the summer.
- Prune the hedge from the bottom to the top, keeping the base thicker than the top to allow light and rain to penetrate. This will keep the hedge healthier than if it were pruned vertically.
- Be careful never to prune wood that is more than 3 years old and do not cut the head of the cedar, it would have a lot of trouble to recover.
- Use a string to obtain a straight hedge.
Is cedar hardy?
The cedar is very hardy (up to -20°C, even -25°C for the Canada cedar).
Beware, however, of certain cultivars with ornamental foliage that begin to suffer at -15°C.
Is cedar sensitive to drought?
Once well established, i.e. when the root system is well formed (which takes about 3 years from planting), cedar is drought resistant.
Can we replant a cedar in the same location to replace a dead or diseased tree?
It all depends on the disease. If it is phytophthora, its oospores and chlamydospores are capable of remaining dormant in the soil for at least 3 years, so it is strongly discouraged to replant a cedar (or any other plant susceptible to this fungus) in the same location.
Why is my cedar turning brown?
The browning of the foliage is a sign that must be taken seriously by the gardener because it can come from several causes, more or less serious:
- Before mentioning the terrible diseases that plague cedars, let's think about the lack of water that can result in scorched and wilted foliage. Lack of water can exist even on watered plants, when they are watered too little. For example, when rain falls but is not abundant enough to penetrate the surface layers of the soil. A mistake that novice gardeners often make is not to water because it has rained. However, it is enough to scrape the surface of the soil to know if watering is necessary! If the soil is dry below the first centimeter, then watering is necessary. Lack of water in the first few years after planting weakens the tree and makes it more susceptible to diseases and pests.
- At the same time, think of amending the soil with compost and organic fertilizers if it is poor in order to avoid deficiencies which will also weaken the tree;
- Another cause, much more formidable: the phytophthora (Phytophthora cinnamomi), a fungus that spreads on the roots insidiously to quickly reach the whole plant. This cryptogamic disease is fatal and contagious. Remove and burn the affected plant and do not replace it with another cedar for 5 years;
- Didymascella thujina, another fungal pathogen, manifests itself as brown discoloration of a few areas spreading in one season to many branches causing leaf drop. Cut and burn the affected parts, water with horsetail manure and often apply treatments with Bordeaux mixture.
- These two diseases are very frequent in heavy and humid soil;
- Foliage that turns yellow and then reddens and dries out associated with blisters on the bark should make us think of cortical canker (Coryneum cardinale), another fungus that attacks cedars with wounds (insects, pruning...). If the disease is caught in time, cut and burn the affected parts, apply a healing mastic on the branches and then treat with Bordeaux mixture.
When to plant the cedar ?
It is best to plant cedar in autumn.
It can also be planted in spring.
When to prune cedar ?
- Maintenance pruning is done in early autumn on trees grown in isolation.
- Hedged cedars should be pruned twice a year: in spring (May/June) and from late August to September/October.
What is the right spacing to create a cedar hedge?
The cedar grows quickly, so don't worry if there are gaps between your trees when you plant them, they will fill in within two years.
By respecting a spacing of one meter between each tree, they will develop in an optimal way by limiting the colonization by the parasites.
Respect the regulation imposing to plant at 2m from the property line the trees that will reach more than two meters in height.
What is the Latin name for cedar?
Thuja is a vigorous, fast-growing conifer in the Cupressaceae family. The Thuja genus includes 6 species native to the temperate zones of North America and Asia (especially China and Korea), however, there are multiple varieties.
The habit of this tree is conical to pyramidal. It has an evergreen foliage generally green with imbricated and flattened scales. The foliage can be toxic by ingestion (vomiting, diarrhea and stomach ache), its contact can cause dermatitis in the most sensitive people.
The flowering occurs in April/May and can cause allergies. The cones are sometimes ovoid, spherical, scaly, sometimes even horned.
What are the species of cedar?
There are 6 species of cedars, some of which are commonly used to create hedges. Let's not forget the taller cedars, ideal for isolated plantings in the garden.
- Thuja occidentalis, is the species most present in our gardens. This tree is otherwise known as Canada cedar and can reach 20 m in height.
- The habit is conical and the bark fibrous reddish brown. There are more than 100 varieties of this species which is widely used to form tall, dense hedges. The varieties 'Yellow Ribbon', 'Lutea', 'George Peabody' and 'Golden globe' have golden foliage.
- Thuja orientalis, or Oriental cedar, has a more restricted development (8 to 12 m high) and a slow growth. Its habit is conical. The foliage is dark green tending towards bronze during winter. This species is well suited to the bottom of rock gardens and beds, or even in pots for dwarf varieties such as 'Aurea nana' with golden foliage, or 'Decussata' with silver foliage.
- Be careful, this cedar does not tolerate wind and needs extremely well drained soil.
- Thuja plicata, known as 'California red cedar', has a pyramidal habit that becomes conical and very ornamental when mature. It can reach a height of 50 m. It is ideal as an isolated tree in a lawn. The variety 'Atrovirens' is perfect for quick hedging. This cedar is drought tolerant.
- Thuja koraiensis, the Korean cedar does not exceed 10 m in height. It can be used as an ornamental in Japanese style gardens for its green foliage with silver reflections and its drooping habit.
- Thuja sutchuenensis is a rare and endangered species native to the Sichuan province in China. It was thought to be extinct until a few specimens were discovered in 1999. This tree can reach 15 m in height.
- Thuja standishii, the Japanese cedar, grows at high altitude (between 900 and 1800 m). It can reach a height of 35 m. It has a conical shape and dark green foliage with silver stripes on the back. It is cultivated as an isolated plant for its ornamental appeal.
What are the best growing conditions for cedar?
The cedar appreciates well-drained soil because it fears having its roots constantly soggy. A deep and fertile soil suits it very well as long as it remains light.
This tree appreciates a sunny exposure but also tolerates half-shade in the warmest regions.
What are the most common pests on cedar?
- Red spider mites can cause serious damage. The leaves turn yellow and then fall off, and tiny webs can be seen on their underside. To prevent them from taking hold, water the foliage every day.
- The bupreste, a beetle whose larvae are phytophagous can condemn the tree in a short time. Oval-shaped holes in the bark indicate its presence. To prevent them, water and spray the foliage with nettle manure.
- Scale insects can weaken cedars. Exochomus quadripustulatus or Rodolia cardinalis, two species of ladybugs, can be used for biological control.
