Mills' role will be to significantly up the nursery's turnover selling plants to garden centres from the current £6m a year. This includes £1m a year within Hillier Garden Centres. Mills was formerly at Landgard and World Flowers.
Hillier's Chris Francis is now wholesale and retail director. He said: "She will assess the market and put a plan together to develop and grow the business to meet the needs of the market. There's no doubt it has changed dramatically. As UK growers there's no doubt we're following behind the Dutch and we need to be leaders again.
"The whole point of our exhibit at Chelsea this year is to make it commercial so the majority of plants on the stand are in our garden centres. It's about our wholesale nursery taking the opportunity from Chelsea. This is very much a Hillier exhibit for all the business - garden centre, wholesale and amenity."
In the first week of May the 12 centres took £1m in a week for the first time, said Francis. That caught up with budget after a "disaster" April and overall sales are eight per cent up, he added. Garden Centre Association (GCA) centres are 2.19 per cent up this year but 10 per cent down in April.
Head of Wisley Retail Ross McEwan said: "May has been better but March and April were slower than hoped. We're very weather-dependent as the vast majority of our sales are plants, unlike most other garden centres. I hope Chelsea along with Hampton Court will give us a boost along with all the TV and media coverage they attract."
GCA chief executive Iain Wylie said on sales: "Everything is looking brighter than it was a month ago. But the likes of seed companies have missed out this year."
HTA president Adam Wigglesworth of Aylett said two weeks ago the centre had a record Saturday and May is "going to come in as a very decent month". But he added: "Though whether we'll make up everything we need to make up an interesting question."
Mr Fothergill's Seeds commercial director Tim Jefferies said: "This has not been a brilliant season for seed." But Crocus co-founder Peter Clay said: "It's been a pretty good year. The weather has introduced different peaks and troughs in unexpected ways. It helps not having the big spike days like the bank holidays garden centres have. We have more spread."