Just a quick tip when it comes to growing healthy plants.
The most important thing that you can do is make sure that your soil is good - with plenty of organic material, well drained and mulched at all times.
So many people see yellowing of leaves on plants, then rush out to buy sprays or fertilizers etc., - when in actual fact the yellowing of leaves - especially at the base of plants is normal, it is just old leaves that can be removed.
When plants seem to be struggling, the first thing you must look at is the soil.
Get your soil right and you will produce healthy plants.