At least once a year, Landform Consultants managing director Mark Gregory will go abroad to pick out stock. "We do some very high-end projects and the developers sometimes only think about the landscape at the last moment. So we may need lots of trees and plants very quickly," he says.
"In Britain we just don't have the amount that we require. In Germany there will be avenues and fields of certain species - often 30 or 40 years old." Gregory will fly over when the trees are in leaf, tag them and get them delivered in winter. "These are big-ticket items. We can spend up to EUR10,000 on a single tree so we want to get it right."
Most landscapers and garden centres will buy at least some stock from overseas, primarily from Holland, Germany and Italy. But they add that buyers have to exercise caution if they are going to get the best out of their overseas suppliers.
A whole range of items is being purchased abroad. In recent years topiarised Buxus has been widely used to provide instant height and greenery to formal planting areas.
Much of this is sourced from Belgium. In addition, they will buy items such as Photinia robina, Viburnum and Amelanchier, which are used in large planting schemes.
John Wyer, a partner at consultancy Bowles & Wyer, also uses foreign stock. "We want topiarised and specimen trees - anything up to five or six metres. We can get individual items in the UK but we have to go overseas for bigger numbers. Foreign nurseries will also do worked trees, such as pleached or multi-stemmed trees.
"They have a lot of Quercus Ilex (holm oak), which we use all the time. If we go to an Italian nursery such as Vannucci Piante in Pistoia, Solitair in Belgium or Lorenz von Ehren in Germany, they will have 30 trees for us to choose from and huge numbers of shrubs."
The trend towards buying foreign trees has been borne out by official figures. Defra has calculated that in 2013-14 more than three million trees were imported - an increase of 25 per cent on the previous year.
Some foreign nurseries have gained an international reputation for specific product. Designer Andrew Fisher Tomlin often specifies using QuickHedge in Holland, which offers instant hedging. "It's very impressive," he says. "It's cut by vehicles guided by GPS. It's a very good product."
Landscapers tend to use herbaceous plants grown in the UK, unless UK stock is low. Willerby Landscapes director John Melmoe says: "Wherever possible we buy shrubs from UK nurseries. If we buy from abroad we use a few nurseries with a proven track record for quality and reliability."
Most landscapers and garden centres believe that customers prefer local products. Woodcote Green Garden Centre general manager Phil Barnden says his firm has a set procedure. "We try to get it locally, then we try within the UK and then internationally. We like to promote British growers."
However, it is not always possible to buy British. Fisher Tomlin says: "We're finding difficulty getting hold of some things that were very common 20 years ago, such as Berberis darwinii or Berberis wilsoniae. We now get them from Holland."
The euro's relatively low price has encouraged buyers to look to Europe. "The exchange rate is very favourable at present, but this has not always been the case," says Melmoe. Like most traders he takes steps to reduce the impact of currency fluctuations. "We manage the risk of fluctuation by hedging the currency on a rolling quarterly basis. We don't always guess correctly but it limits the pain."
There are practical issues with buying abroad. Buyers often tag the trees or large plants with their own personal tags to ensure they get the trees they asked for. They will often take photos of the tree, with somebody standing next to it, so they can see how big it is. The picture also useful if the plant is damaged at any point.'
Gregory offers some useful advice - discuss transport arrangements, find out how many plants you can get on a truck, be careful about buying out of season, do not unload the lorry until you have thoroughly inspected the load and always take a photograph before you unload in case there is any dispute about when damage occurred. Also, make sure you have the right personnel to unload.
It is important to build up good relationships with suppliers. They will learn exactly what the buyers want in terms of price and quality and may be able to suggest plants to meet their needs.
Most landscapers will not bother with the big trade shows. "We just don't have time," says designer Andrew Wilson. However, they will often buy stock through an agent.
Stephen Page owns Walter Blom, a Dutch firm that supplies plants, primarily to the retail trade. He has some tips for building a good relationship. "You have to know exactly who you are dealing with. If they are agents, they might not have such an intimate knowledge of the plants. You have to discuss the transport costs - is it ex-warehouse or delivered to your door. Will it be unloaded?" Unless there are good relations, suppliers might be tempted to give preferential terms, and the best plants, to the larger and more lucrative customers.
Another is issue is that plants purchased in hotter climates will have to acclimatise. "We buy trees from Italy," says Wyer. "After they've been planted they start to look a bit sick. A tree can lose two-thirds of its leaves in three months. Because there is less light in the UK, the central branches will defoliate. It will then refoliate in a more open way. Often we will keep a tree in a UK nursery to give it a chance to adapt. Otherwise the customers might think we were selling poor-quality trees."
Fisher Tomlin says he always tries to buy plants from Holland and Germany rather than Italy because the plants grown in the harsher conditions of northern Europe are better able to withstand the British climate.
There are also issues of disease. However, most buyers suggest that by following the law, ensuring plant passports are issued and buying from reputable firms will minimise risk.
Long-standing relationships key to sourcing for garden centres
Garden centres have always bought stock from abroad. But in recent years, a cheap euro has seen overseas purchases increase. The vast majority of houseplants are sourced abroad. Other plants garden centres might purchase from overseas include "impulse bedding", topiary, bay and box trees, and Skimmia, Photinia and Viburnum.
The attitude towards agents varies. Hilliers avoids them because the quality can be "hit and miss". However, most centres have built up long-standing relations with agents and suppliers and see them as valuable business partners.
Phil Barnden at Woodcote Green explains that he buys from a variety of sources. "We go to big suppliers such as Javado in Holland or Europlants, the British importer based in Hertfordshire. We get a regular van sale from Plantline from Holland. This is great because it means that we can get some colour straight onto the racks."
He has built up relations with these traders over periods of 10 years and more. "We need quality and consistency," he adds. "If you've built up a relationship, they won't sell you something that has been on the back of a lorry for a week."
Squire's purchasing director Darran Oakley says deterioration of plants in transit is not a major issue. "These days transport links are very good and trucks are refrigerated."
Europlants managing director Renato Canale acts as agent for Vanucci Piante as well as supplying plants from other overseas growers. "Because we get our stock from Europe, we have a huge selection and we're known for quality," he says. "We work very hard to maintain our reputation. If there are problems we have to explain them to our customers in advance or they won't forgive us."
Although relatively few landscapers will go to the international trade fairs, many of the retailers will visit them to seek out new plants and identify trends. Buyers from Squire's usually visit Plantarium in Holland as well as some of the big German shows. "We try to find new suppliers. We will then put them in touch with our existing wholesaler so he can arrange for the stock to get to us," says Oakley.
Eastern European firms are now trying to get into the UK market. Although most garden centres like to stick with their tried and tested suppliers, many are starting to look carefully at the Eastern stock.
But no matter who they buy from, garden centres have to be careful. Stephen Page of Walter Blom advises: "Always check what is included in the price. Does it include colour labels and trays? If you can't visit, ask them to send a picture. I like to visit the nurseries a couple of times a year. I want to see that they are clean and well managed, or their stock might be variable and they might spread plant disease." Even in the computerised age there is no substitute for the personal touch.