He says gardeners must be aware that they may need pesticides to control pests such as whitefly and points out that they will not be able to produce show-standard crops on a budget.
"We need to send a message as professionals about growing your own. The message needs to be more honest. Don't worry if you don't get produce that is show-standard. Some crops by their very nature are in need of chemical control," he says.
"The problem most people ask me about is: 'How do I prevent greenfly?' I tell them I spray. And about things like companion planting, we should be more honest and say: 'Look, it can work but with just one marigold if you expect it to attract all the greenfly on the allotment it is not going to work.' But people believe it will. It's our fault. We should be more honest. That's how I see it, speaking for myself and not the RHS.
"Next year I will grow fewer brassicas because of the whitefly problem here. I don't like spraying, but these plants are extremely susceptible to all sorts of pests. I'd rather play safe and steer clear than spray three times a week."
De Pace says grow your own costs can be more than people expect, too. "I don't count the costs because I'm not really tied by budget at Wisley, but if you build in the chemicals, fleece, fertilisers, containers and supports, one kilo of tomatoes can cost £100 for the beginner. That's the reality," he insists.
"You have to be honest with people. You can't say to them you can grow anything anywhere. Veg growing should be about enjoying what you grow, even if it is just a few tomatoes.
"But people have high expectations. They go to shows and see perfect veg and they think: 'If I buy this, this and this, I can do the same.' But it doesn't work like that. Sometimes the information given out is too technical. Some people are getting a bit disillusioned. The people benefiting from grow your own have become plant centres and seed companies."
Having started at the Surrey garden in January, he says it "was a shock because half my time was taken up answering questions from the public. There is an amazing interest in growing vegetables.
"The majority of questions has been about runner beans, because I sowed them in mid July. Everyone asks: 'Are these late?' People beginning to grow their own come with a notepad to write what fertiliser to use, how to prepare the ground, what variety - they want to know how to get started."
The main pest and disease problems include whitefly on cabbages - "I've been trying the mildest spray to the more toxic but I still can't get rid of them" - and blackfly on broad beans. "I've been quite successful on blackfly. People were wondering how I kept the broad beans clean. I used Phantom."
De pace took an unusual route into the Wisley role: "I've been at Wisley eight-and-a-half years. It was a change of career. I wanted to do something less stressful than working in a restaurant. I couldn't cope any more with long hours. I spent most of my restaurant life in the kitchen and that's why I decided to find something less stressful outside.
"I started gardening late in my life so there are so many things to learn about growing plants, and veg specifically. It is a lifelong learning curve. I used to visit Wisley and a I took a job here by chance in January 2002. After going to the orchid show I went to the plant centre and there was an advert for a general assistant. I applied and got the job.
"Then there was a position in the garden and I applied in the trials department. The condition was, because I had no formal qualifications, that I started as assistant gardener and promised to undertake the RHS general certificate."
He ended up in the model vegetable garden after the RHS "went through a lot of change last year". He says: "The previous veg gardener didn't want to accept the new conditions (weekend working), so I took over. I applied because of my background in restaurants and interest in veg growing. The old guard used to grow plants to show standard. I grow to the best of my knowledge but keeping in mind the taste."
1975-2002: Waiter, wine waiter, restaurant owner
2002-2010: General assistant rising to gardener, Wisley Plant Centre
2010 to date: Model vegetable garden horticulturist, RHS Wisley