One of the facts of life in this business (maybe all businesses) is that of repeating questions and the most common questions we get are about watering. Sometimes people don't even know their question is about watering!
In training our own staff to water the most important instruction we give is that you have to make a judgment first before you water. I am sure (I hope!) that at the outset this appears to be a daunting task. Every plant in the garden centre must be checked and a judgment made about its level of dryness along with factoring in how long it will be before you might be able to check them again. Once that decision is made the next part is easy …. apply sufficient water to saturate the soil.
How do you know when the soil is dry enough to water? That comes with experience but our rule of thumb is the water them just before the point of wilting. As you observe the reaction of any plant as the soil dries you will be able to predict that point. Like so many things in gardening their is no substitute for experience. Judgment > apply water if needed > repeat.
The reason that you need to let the soil dry out is that the roots need oxygen derived from the air. Water holds less oxygen than air and if the roots are saturated for too long a period of time they will start to die from a lack of oxygen.
If you let the plants dry out to the point of wilting for too long a period of time you will start to desiccate the fine root hairs that are so valuable in increasing the surface area of the roots. Back to my our own staff. In training we always take plants out of pots and look at the roots with a magnifying glass (see image from above). When you do that you will see many fine hairs and it is very easy to see how they could be susceptible to being damaged from drying out.
Once you damage those fine root hairs from either over-watering or under-watering the result is the same. The plant no longer has sufficient root system to support its top growth. As with many statements of "fact" there is a qualifier. Plant top growth tends to match the root system other wise the plant would not be able to take up enough water and nutrients. It seems like common sense but in fact conditions can arise that will make a plant ill-prepared for the conditions it is facing. If a plant has come out of a greenhouse it has likely been a very humid environment. Once moved outdoors it will be subjected to very different conditions. The plants root system may not be prepared for the differing conditions and the plant may wilt even with wet soil. Remember my rule before was a "rule of thumb" ….this is the exception. What you can do to help the plant adapt is give it some shade and/or a bit of a misting to increase the humidity around the plant so it is not giving off as much water from its foliage (transpiring). Once it has larger root system capable of doing the job your special treatment can end.
What is the most common question we get? " How often should I water it?". If you understand what I have written above then you know that the answer to that question is a complex one and that you cannot water on a schedule. You should check your plants on a schedule . I think Yoda said in Star Wars "Water not Luke, check"….or words to that effect!