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.
