Why are the Thuja Green Giants that I planted in the fall not doing well?
Question:
I need assistance determining why my trees are not surviving in the ground. I planted 20 thuja green giants (5-6 feet) in early October 2024 and four are completely dead. Six are more than 50% dead. The other ten are doing okay but not thriving. I’d love to test the soil and see what can be done. The odd part is there’s 3 or 4 that are doing incredible.
Answer:
The primary reasons most residential plantings of young arborvitae (Thuja occidentalis) have a number of dead trees are 1) the planting preparation and installation, and/or 2) the care regimen after they go in. This isn’t to rule out the possibility of a soil issue or a pathogen, but this is the lion’s share of these premature deaths.
Our guidelines for planting new woody plants recommend wide holes for each plants (roughly 3x the root ball width, or at least 12″ for bigger root balls), shallow depth (setting them too deeply, even by a couple of inches, can cause root rot problem and tree decline), and watering them very well through the season (stopping a couple of weeks before the first frost). If the site of the planting is in a low/poorly drained area or has insufficient light, even proper planting might lead to the same problem through excess soil moisture. Arborvitae prefer consistently moist, well-drained soil, and will do best with 6+ hours of direct sun per day.
Winter care is very important, as well, for young plants. We recommend a flat 3″ layer of organic mulch across the root ball, not quite touching the bark and not mounded up like a volcano. Arborvitae will also benefit greatly from wrapping with burlap before winter to give them an extra layer of insulation from the wind (often the browning you see on these plantings are “winter burn”). These measures are even more important in years without a heavy snow pack to insulate the crowns.
If you conducted the planting and care, you’ll be able to tell how closely your approach was to the optimal advice. If a landscaper took care of them, I would reach out to inquire about it (and many have a replacement policy for new plantings). Either way, this is how I would recommend proceeding with any new plantings.
With these just going in last October, I suspect they just didn’t fully acclimate to be prepared for cold weather, and you’re having some winter kill. If the holes were prepared well and you did everything else per the suggestions above, I will dig a bit deeper into other potential causes.
It is a good idea to get the soil tested (ideally, this happens before the planting so you can easily make amendments that might be needed, but it’s perfectly fine to do it now) through the UMaine Analytical Soil Lab. You can stop into any County office to pick up a test kit and instructions, or ask the lab to mail them to you directly. This will help you identify whether your soil has plenty of nutrients and organic matter, as well as give you the soil pH, which is just as important. While arborvitae will tolerate a fairly wide range of pHs if not otherwise stressed, pH governs how easy it is to take up nutrients from the soil even if they are present in sufficient quantity.
As for why 3 or 4 of them are thriving, that’s just one of the wonders of biology. Plants have variable capacities to withstand things like all organisms, and you’ve got some that are resistant to whatever hit the rest. You can also have little microniches of better conditions that will sometimes support those plants, as well.
Happy gardening.