I suggest laying off your daily watering regimen. You could easily be contributing to a happy humid home for unwanted pests and fungus.

So long as you have good drainage, the plants will take what water they need. General rule of thumb is that gardens require one inch of water per week by supplementing rainfall.

You will have to adjust throughout the season and be attentive to the amount of rainfall you receive each week. There are two schools of thought on watering. Frequent shallow waterings or less frequent long, deep waterings. The general rule of thumb is that mature plants need 1-2 inches of rain per week in summer. You should provide whatever portion of this moisture is lacking. One inch of rain equals 65 gallons per 100 square feet of garden space. But this is just a guide. Time of the season, stage of the plants, soil composition, and many other factors affect the amount of water required. Be consistent during fruit production.

Mulched plants require less water than plants that are not mulched. Raised beds require more than non-raised beds.

If a good amount of compost is incorporated into the soil, less water is needed. Compost holds 6 times it's weight in water.

Watering too frequently and too heavily is just as hard on plants as too little water. Roots require oxygen just as much as they require water and nutrients.

Dry winds dehydrate plants.

Cooler temps require less water.

Those using drip irrigation whether from a soaker hose or milk jug are likely getting the best use of their water which is going right where it needs to go and the ground has time to absorb it.

There are hundreds of methods of watering. Whatever you do, just be consistent. But I do agree, twice a day sounds like too much. Roots require oxygen equally as much as they require nutrients and water.

I personally believe in a couple of deep waterings per week with the use of 2 inches of mulch to reduce evaporation and maintain a more stable soil temperature.

Plants recover from dehydration much better then they ever recover from the roots being suffocated by drowning so under watering is better than over watering until you get the hang of it. Oxygen is equally as important as water and nutrients and rain water is always better than municipal tap water.

Now for your "blackfly" problem which may or may not be the problem. Take the time to learn that there is not a "cure all" for every pest at every stage of it's life cycle. Some won't work if it's going to rain. Some don't work well if it's too hot, etc.

Pest management is complex, if you wish to use chemical products, do so responsibly which goes further than reading the label. If you want to learn, a good place to start is here:

Integrated Pest Management
This lecture is presented in two parts. Each part is 90-minutes in length. Recorded in Sacramento County in California's Sacramento Valley, this lecture is by Mary Louise Flint, Ph.D., Director, IPM Education and Publications, UC Statewide IPM Project and Extension Entomologist & Cooperative Extension Specialist.

Education:
B.S. Plant Science, University of California, Davis
Ph.D. Entomology, University of California, Berkeley

Appointment:
100% Cooperative Extension

Research Interests:
Integrated pest management of landscape, agricultural and garden pests; biological control of arthropod pests; alternatives to pesticides; adoption of alternative practices by practitioners; innovative delivery of pest management information.

Topics discussed in the Integrated Pest Management Lecture:

* IPM references and resources
* Preventing pest problems
* Natural common enemies
* Making less toxic pesticide choices
* Controlling aphids, scales, caterpillars, coddling moths, tree borers, snails and slugs, and lawn insects.

You can watch the programs now online:

Just make sure you have Real Player installed or download it free.

Integrated Pest Management Part1 90 minutes

Integrated Pest Management Part 2 90 minutes

You'll want to bookmark the following link to Professor Flint's Lab Research on:
Controlling Pests in Gardens and Landscapes: Vegetables and Melons. This is where you can look up "gnats" which are your blackfly.

I promise you'll learn one or two things to put in your gardening bag of pest management arsenals.

;)

Hi Mike, what black fly do you mean? a tiny one that flies annoyingly around the surface of the soil, especially when watering? Is the soil (or has recently been)kept soggy?

GMO

I see you live in the UK, where the citrus blackfly is a very serious pest (if that's what you have). Apparently it's showed up only in Florida in the US, so not many people in the US have experience in dealing with it.

What sort of compost is New John Innes? One site suggests that blackfly larva are often found in compost.

I found this UK site where some cannabis growers are discussing how to get rid of it! Don't know if any of the suggestions are useful or not, but I couldn't find much else about identifying it or the damage it causes or how to control it. Maybe someone else here will have some suggestions.

Mike, can you obtain Neem oil in the UK? It proven to be very effective against blackfly (aphids) on veggies and other plants. Neem oil is especially helpful as it works biochemically as an anti-feedant against those insects that feed on plant tissues and/or sap.

Insecticidal soaps applications can also help you get control of a bad aphid outbreak. Both of those products are harmless to non-target insects once dry.

Though I am not personally familiar with John Innes compost products, I believe that they are composed of sand, loam, compost and other products that sometimes don't work very well within the confines of a container. You see, though those ingredients work well as part of a much greater in situ soil system, they don't behave well 'in captivity', so to speak.

When we grow something in containers, it is imperative that we do everything that we can to provide OXYGEN throughout the volume of the growing medium. We can do so by incorporating coarse peat, perlite, composted bark fines, and other materials that I won't mention because I have no idea if they are commonly found over there or not.

I would be willing to bet that the yellowing leaves are caused by the 'muckiness' of the compost mix you are using....due to the plant's inability to have ready access to oxygen.