Why thuja turns yellow: identify the cause before treatment

Yellowing inside the plant or on outer shoots

Part the branches and identify which area has changed colour. On a healthy thuja, outer growth can remain green while the oldest inner foliage turns yellow or brown and falls in autumn. This seasonal renewal does not automatically call for spraying the whole plant. However, stressful conditions can increase foliage loss, so assess the planting's overall appearance as well.

Yellowing branch tips, large sections or the entire canopy presents a different situation. The symptom's name alone cannot explain the cause. Inspect the plant from every side, compare it with neighbouring thujas and recall when the changes began. A uniform pattern along the entire row and one damaged plant among healthy neighbours provide different starting points for investigation.

What to photograph for comparison

• The entire hedge or group of thujas from the garden side.
• A whole individual plant, including the lower canopy.
• The boundary between green and discoloured sections of a branch.
• Inner foliage, outer shoot tips and the soil surface around the base.

Keep the dates and repeat photographs from a similar distance. This helps show whether changes are spreading or remain within an existing damaged area. One close-up of a yellow branch cannot reveal what is happening around the roots or across the rest of the plant. For advice, provide the full set rather than only the most worrying detail.

Check water and conditions after planting first

Thuja foliage can discolour when the roots are too dry or the soil stays excessively wet. Newly planted roots still occupy a limited volume, so check the original root ball as well as the surrounding ground. Moist soil nearby does not always mean the root ball has enough water. Conversely, repeatedly adding water without checking can worsen waterlogging.

Consider whether irrigation changed, prolonged rain or a dry period occurred, or work took place near the hedge. Look for areas where water collects after rain. Avoid prescribing the same water volume for every thuja regardless of age, soil and weather. Establish actual conditions before changing the care routine, and keep observations for the affected plant separate from assumptions about the whole row.

Why thuja browns after winter or trimming

Foliage loses water in winter while frozen soil can limit replacement through the roots. Sun, wind and earlier drying affect the risk of winter desiccation. In spring, avoid immediately cutting off every brown tip before it is clear whether living buds remain and where growth resumes. Repeated inspection helps distinguish visible foliage damage from the death of a branch.

After trimming, also check the depth of the cuts and conditions during the work. Regrowth from bare old sections of conifers is unreliable, while heavy shaping during drought or heat adds stress. If the problem followed such work, do not immediately assume a fungal disease. Prepare a description of the pruning and photographs to help investigate the cause.

When to investigate pests and diseases

Patchy yellowing and browning may occur alongside webbing, small organisms, sticky deposits or black structures on affected parts. These observations can narrow the investigation, but one matching sign cannot establish a specific cause. Small pests and fungal damage need careful examination and sometimes professional diagnosis. Record the signs you actually find rather than selecting a product solely from the colour of the foliage.

If damage spreads, branches visibly die or the whole plant weakens, seek clarification promptly. Selecting a plant-protection product requires an identified cause, a permitted plant use and the appropriate instructions. Do not assemble a mixture of products based on foliage colour or treat spraying as a universal response to every kind of yellowing.

Were nearby areas treated?

Include any weedkiller use near the thujas in the history of the problem. Accidental exposure can be associated with browning or distortion that resembles other damage. Recall where and when the work took place, which product was used and whether foliage changed afterwards. Investigation needs the actual product name and application details, rather than a general suspicion about chemicals near the fence.

Timing alone does not establish cause: noticing yellowing after a particular job does not prove the job caused it. Compare the damage pattern, neighbouring plants, water conditions and other observations. Mention suspected accidental exposure when seeking diagnosis. Avoid trying to neutralize an unknown effect with a mixture of other treatments or an extra portion of fertilizer.

Before a consultation, write a short chronology: when the thuja was planted, when it looked healthy, when the latest work occurred and when changes began. Distinguish what you observed from what you only suspect. Add the product names from their packaging and your photographs. This information helps identify a useful next check and avoids starting several unrelated treatments in succession.

Where fertilizer fits into care

Consider feeding after assessing the soil, water and previous applications. If a nutrient shortage is suspected, check both the site's supply and why nutrients may not be reaching the plant adequately. Magnesium or another nutrient listed on a label does not prove that a shortage of that particular element caused your thuja's colour change.

If the checks show a need for feeding, compare fertilizers for conifers by their instructions, application period and the size of your plants. Buy the amount needed for that planting rather than extra fertilizer simply because the foliage is yellow.

Conifer fertilizer: when feeding is appropriate

The range includes Complex fertilizer for coniferous evergreens, granules for feeding conifers and ornamental trees and shrubs. Pack sizes are 1, 3 and 4 kg. These are dry granules, including when the packaging resembles a bottle or canister. Choose the pack according to the calculated need rather than assuming its entire contents belong to one plant.

Application methods and periods from the instructions

When planting plants
As the main fertilizer, it is used in the spring when planting and feeding plants. The granules are spread over moist soil. The application rate is calculated depending on the maturity and size of the plants: for young plants - 50-60 g/plant, and for mature 60-200 g/plant.

During the growing season
It is applied superficially on wet soil with subsequent loosening. Fertilization is carried out 1-2 times during the growing season, depending on soil fertility and agro-climatic conditions. With surface application of 50-200 g/plant, or for every 10 cm of conifer height, 5 g of fertilizer evenly around the stem circle.

The instructions describe different conditions and calculation methods. Do not add the height-based quantity to the per-plant quantity as two compulsory portions. Consider the thuja's size, maturity and condition when selecting the relevant instructions. For dwarf cultivars, read the product's separate guidance. If the plant's condition remains unclear, establish its need first.

Compare other options on the conifer fertilizers page. Check the complete instructions and permitted period for the selected product. Feeding cannot replace drainage of excess water, identification of a pest or treatment of a confirmed disease.

How to keep observations for a hedge

Save the thujas as a separate area in My Garden. If some plants are young and others have been established for years, separate them when planning. Record the plant count and dimensions required by the instructions. Do not automatically use one average value for differently sized thujas without checking whether it is appropriate.

In your own care notebook, record the date yellowing appeared, irrigation, rainfall, trimming and products used. Record other work near the hedge too. Review the entries before buying again: they help avoid duplicate applications and explain what has already happened to the planting. Compare the same areas of the canopy across successive photographs rather than judging different branches each time.