I have several Portuguese topiaries in pots and growing in the ground. They were all planted at the same time, but my pot ones are not doing well. They have yellow leaves and black spots. I sprayed them several times, but they are not healthy. I was wondering if you could please advise.
Plants grown in containers do not have access to nutrients and water as readily as those grown in the ground. Therefore, they require a little extra care and attention. From the sounds of it, your Prunus lusitanica (Portuguese laurel) are struggling in their containers. This could be the result of poor nutrition, poor drainage, or the plants are root bound (roots have filled the pot with no space to grow), or a combination of all three factors.
Investigate by removing one of the plants from its container to check whether it is root-bound and if there is a lack of drainage. Both issues can cause yellowing of leaves and black spots.
Replant preferably into larger sized containers that have ample drainage using Daltons Garden Time Outdoor Container Mix with some Premium Tree and Shrub Fertiliser applied to the surface. Regularly water your potted plants but be careful not over water them either. To test, stick your finger about 3cm into the soil, and if it feels dry, water your plant.