I planted my Emerald Green Arborvitae (Thuja occidentalis) in a sunny spot 3 years ago. My soil is not great, with lots of clay, but I try to make it more drainable every time I plant something. It enjoyed healthy growth in those 3 years, with no problems. This last winter the top of the thuja dried/browned out. The browning took about 20 cm.
Do you suggest I cut it?
And what can I do to prevent this from happening again?
Browning Emerald Green Arborvitae or Thuja occidentalis, is a fairly common concern amongst evergreen gardeners. There are many factors which can lead to plant stress and this resultant browning. You can prune your tree in early spring, but it may green up again if the cause of the browning was water or nutrient related.
If your plant has done well over the last few years it is likely getting enough sunlight and adequate nutrition. Your soil clay condition is not ideal, given that these cedars prefer well drained soils.
Arborvitae have shallow root systems which make them susceptible to heat and water stress. They don’t like sitting in water (which can happen in slow draining clay soils), but also suffer with desiccation (browning of the foliage) if they overwinter without enough water.
To improve your soil drainage, add a bucket full of well-composted organic matter annually per tree. You can add it like mulch, so not to disturb the shallow roots, and it will continue to break down over the year, percolating down into the deeper soil layers over time. This is important to help improve the structure of your soil and add nutrients.
If you ‘feed your soil’ with organic material such as compost, well rotted sheep manure or leaf mulch, the soil will feed your plants and help to protect them from the fluctuating weather conditions we’ve been experiencing.
See our Gardening Guide: https://184.108.40.206/~torontom/wp-content/uploads/2012/07/Improving-Your-Soil-Organically.pdf.
Mulching around your plants (trees and shrubs) with any organic material such as chopped leaves, straw, pine needles and bark chips throughout the growing season will not only conserve moisture, but will gradually improve your soil structure. Simply raking your leaves, chopping them with a mower and then covering the exposed soil areas with them will be a first step in feeding and improving your soil.
There are several species of fungus that can cause browning from the top of the tree down, but if you remove the stressors from the picture (as above), your tree can improve and regain a healthy state. In your case, this probably means improving your soil structure- increasing your drainage, a few boosts with nutrients/ fertilizer and adequate water between now and winter.
For further reading, here is a link from my peers with useful watering and fertilizing advice: https://www.torontomastergardeners.ca/askagardener/dying-and-browning-emerald-cedars/