"Both plants are watered when necessary with a regular fertiliser." Please explain how you determine when it's time to water, and what you mean by the statement. Are you saying you fertilize every time you water?
1) "Could the temperature shift back and forth have affected both plants?" Doubtful. The only think I can think of is if the pots were directly exposed to sunlight and root temperatures got hot enough to kill some roots. Did your pot/plant blow over and then become exposed to direct sun. Blow-overs are much more susceptible to damage to roots via over-heating.
2) "Could the Thuja be underwatered? It could be. How do you determine when it's time to water? Do your pots have drain holes? If you're watering while you can still see or readily detect moisture in the medium, you're almost certainly not over-watering. Unfortunately, over-watering produces the same symptoms as under-watering; this, because excess water in the medium limits root function or damages root health, making it difficult for the plant to keep its most distal parts fully hydrated. If you ARE watering while you can still readily detect moisture in the medium, odds are much higher that you're over-watering. Does your home have a water softening system that uses salt as the exchange medium?
3) "Is the Thuja showing obvious signs of nutrient deficiency?" Nothing definitive. Tell us about how you approach nutritional supplementation/ fertilizing. What product(s) do you use? Please include the NPK %s. If the plant is being kept indoors, it's a good bet that's part of the problem.
"..... the Gardenia we added Epsom salts because we read in several places that it was helpful for Gardenias to provide minerals and to try to avoid yellowing." It's generally poor form to attempt fixes aimed at supplementing only one or two nutrients. There is an IDEAL ratio of nutrients that should be provides. This is done best by making sure you're watering correctly, so you're flushing the soil as you water, and you fertilize regularly with a fertilizer product that provides nutrients in a ratio as close as possible to the ratio at which the plant actually uses the nutrients. By increasing the level of magnesium (Mg) w/o knowing if a Mg deficiency actually exists, you run 2 risks. 1) Adding Mg when there is no deficiency unnecessarily raises the level of TDS (total dissolved solids/ salts), which makes it more difficult for the plant to task up water and nutrients. 2) A toxicity has the same ability to be limiting as a deficiency, and an excess of Mg in the medium limits uptake of potassium and calcium while increasing uptake of phosphorous.
It's far better to flush the soil each time you water by making sure the entire root/soil mass is fully wetted and at least 20% of the water applied exits the pot's drain hole, then fertilize about every 3rd or 4th time you water during active growth periods. I have many acid loving plants in pots and I use Foliage-Pro 9-3-6 as my 'go to' fertilizer for everything I grow in pots.
It has all nutrients essential to normal growth in a single package, and has a list of additional attributes very few fertilizers can deliver.
Questions?
Al
