The willow comes from the family Salicaceae which includes the poplars.  The willows (salix) are very common growing all over the northern hemisphers.  There are over 300 species of willow with many more varities.  In basketry the most common species used are Salix triandra, Salix purpurea and Salix viminalis although many other species can be used to make baskets.  The willow rods (withies) are long,  tough and pliable which makes them very suitable for this purpose.  They are very easy to grow and take readily from cuttings. 

Although they can be grown from seed the easiest way is to plant cuttings.  The cuttings should be about one foot long (30 cm.) and should be planted about 9 inches deep (23 cm.) with perhaps two buds above the surface  and about 18 inches (45 cm.) apart.  They should be planted in spring in well prepared soil which you have to try to keep weed free for the next couple of years or so.  Weeding is really the only attention they require but they must be cut back every year close to the ground or as pollards (see below) in order to produce a lot of long fine rods suitable for basketmaking.  Once established they will grow quite happily for 20 years or more.

Left: willows cutting about 4 weeks after planting
Centre:  Willow cutting 3 months after planting
Right:  Willow (salix viminalis) grown as a pollard about 8 feet high and used as a fence post.  The top of the fence is about 6 feet high the vetitation below is bracken

Normally willows for basketmaking are cut close to the ground ( coppicing) but they can be grown as pollards which can be anything from a foot to 10 foot high.  The main reason for  growing as a pollard is to keep the young growth out of reach of browsing animals (rabbits, deer etc.) and also to keep it above the weeds.
Willows like a moist rich soil but don't  require much aftercare or artificial manures.  They drop their leaves every autumn and these provides sufficient humus to sustain them.  The first few years are the most critical when they can be overwhelmed by weeds.  They will not do well if there is a drought during the first year when they have little root growth so it might be a good idea to water them during dry conditions.

Harvesting is  done in the winter months when all the leaves have fallen and the plants are dormant.  The rods can be cut with a billhook or secateurs after which they are usually graded according to their length.  The rods can be used for some basketry at this stage (green willow) although if you want to make living structures then it might be better to leave the cutting till later in the year (early spring).  This "green" willow will tend to shrink as it dries and baskets made from it  tend to become loose as they dry out.  Hanging flower baskets can be made in the green stage where tightness of the weaving is not important and the fact that the willow will sometimes grow can be a bonus.  This green willow can also be used to practise techniques.  Most of the willow rods will be processed into material suitable for basketry. 
Brown willow:  This is the name giving to willow rods that have been dried.  It is possible to use  the willow after only a partial drying.  In this case the rods are stored in a dry place for a few weeks or even months until they "cling".  They are said to cling when the bark become a bit wrinkled and can be used at this stage without further soaking.  If the rods are dried out completely (which is more usual)  then they must be soaked before use.
White willow:  In this case the willow is placed with the thick end (butt) in water and allowed to sprout new shoots.  It is then pulled through a brake to remove the bark and then allowed to dry (see stripping willow)
Buff willow:  This is produced by boiling the willow for a few hours and then removing the bark. 

Brown willow can be used when partially dry (see above)  but if it is completely dry it must be soaked and mellowed before use.  Soaking and mellowing times vary depending on the  thickness of the rods and the variety of willow used.  Generally the rods must be soaked for about a week in cold water and then wrapped in a wet blanket for a day or so to mellow.  This process can be speeded up if necessary by using hot water and that applies to all willow rods brown, buff or white.  If you don't have a container big enough to soak the rods then you can wrap them in a blanket and pour boiling water on them at intervals throughout the day.
Buff and White willow require less  time soaking.  This can be as little a one hour to a day depending on the thickness and variety used.  Once again the rods will benefit from mellowing in a damp blanket for a few hours.