A representative said Bridgemere could supply a large proportion of the shrubs the 120-centre group needs, while Woodland could provide poinsettias and the new Cardiff Castleton site will grow a range of plants for the group when it has been turned into a nursery this winter.
Chief executive Nicholas Marshall said the "demand for British-grown plants" was driving the move. The strong euro and the desire to use spare land has also prompted the change.
The grower charged with building up the proportion of plants the Garden Centre Group grows for itself, former Notcutts grower Paul Masters, has left and been replaced by Wyevale regional store manager Alyson Haywood, the former Woodlands centre manager.
Marshall added that autumn bulb sales had started early, while he presented 134 long-serving Wyevale staff with awards last month at Syon Park in London.
Meanwhile, the Garden Centre Group sources suggested "95 per cent of growers are happy" with the company's dealings with them despite new payment terms that appear to remove a supplier's right to retain title to their goods until the invoice has been paid. Another term would prohibit a supplier from withholding goods when overdue accounts remain unpaid.
Marshall, who denied that trading terms had become tougher for suppliers, will speak at the next Garden Industry Manufacturers' Association meeting on 7 October at Henley in Arden, Warwickshire.