When tying growing media to a living branch of a tree keep in mind that the branch will grow and increase in girth.If the plants you have planted in the media do not have extensive root or rhizome systems (Bromeliads are a good example of plants that do not have large root systems) then as the branch grows it can split the media apart. If this happens simply add more media to the split and retie with string. To avoid this happening in the first place plant an epiphytic fern such as Davallia species with the bromeliad as the ferns rhizomes will encircle the branch again and again to the point that the ferns rhizomes become a growing media in themselves. Brom in fern Alternatively plant an orchid hanging from the bottom of the clump of plants on the branch as the orchid roots will also encircle the branch and keep up with the expanding girth.
If you are growing epiphytes outside in a tree it helps to have a plant or plants which are not true epiphytes that will begin to wilt before your epiphytes are totally dry (fuchsia are a good example but ensure they have a good depth of growing media) These plants act as an early warning system if they are begining to wilt then every plant in the tree will need a thorough watering. This is an excellent system for very busy gardeners as a quick glance at your early warning plant as you rush past will tell you if you need to add watering to your busy schedule or not.
Be careful which plants you group together.Bromeliads for instance often have spines on the leaves and when the wind blows they will saw through an epiphyllum cactus leaf if the two are planted together. Small, low or very slow growing plants will be overrun and smothered by larger more rampant growing plants .For instance a small coelogyne cristata orchid cannot compete with a davallia mariesii fern.
An excellent growing media for anything you might want to grow up a tree (Epiphyte or simply a plant that can handle dry conditions) is the old half rotted bases of phoenix canariensis palm fronds Palm (If the frond bases are not rotten enough you may have to remove the outer layer to get to the soft porous inside) either tied on to the branch whole in groups (3 or 4 trimmed to fit around the branch which is ideal for orchids) or crushed up coarsely,mixed with some bark (Orchid mix) and held in place with coconut fibre tied with string but leave some larger pieces on the top side of the branch to prevent it all sagging to the bottom (this latter method is a good way to get a reasonable depth of media for non epiphytic plants such as phormium,agapanthus or fuchsia). Simply poke holes through the coconut fibre and insert your plant. If mounting a single plant the matted fibre from the trunk of a windmill palm palm (Trachycarpus fortunei) tied with string can be used to hold the crushed palm frond bases and bark in place. When planting into this media the addition of a small amount of sphagnum moss around the roots will help the plant establish quickly and the plants roots will bind all the media together.
Always thoroughly water any new planting and then keep moist untill established. Once the plant is established only water as required (remember that epiphytes need to almost dry out between waterings) Frequency of watering depends on your growing conditions.
Ultimately in an outdoor situation the growing media will break down and become humus caught up in the roots of your epiphytes. This is not only natural but desirable as the end rusult is a perfectly natural looking mass of plants growing all around and along the branch Epiphyte branch The larger clumps are also perfectly natural looking. In nature you will get a moss/fern mat growing along a branch with larger plants such as astelias,orchids,bromeliads or birds nest ferns growing out of the mat at various intervals Epiphytes in situ. Other hanging epiphytes in turn grow out of the lower portions of the larger epiphytes. All of these epiphytes on the branch collect various organic bits and pieces (Twigs,leaves,dead insects,feathers,droppings from inumerable organisms ranging from the microscopic to the largest bird that perches in or flies over the tree or even from animals that may choose to climb the tree) and of course bird nests, the latter are a mixed blessing though as some smaller epiphytes may be smothered by the nest and some birds like to use bits of plastic bag and other non organic rubbish in their constructions. All of this will go on (mostly unseen) in your epiphytes and so once you have got them growing as long as you provide adequate water when necessary they will continue to add to the humus on the branches,continue to grow around and along the branches and of course increase in size and multiply.
Many bromeliads produce their brightest colours in full sun but if you get frosts in the winter please remember that full sun in summer equates to full frost in winter. Therefore careful placement of your bromeliads is important. More shade from overhead evergreen trees = less colour but more frost protection. The more frost hardy cultivars / species can be placed nearer the edge of the overhead protection or lower down (Cold air always goes to the lowest part of your garden = more frost) and the less hardy ones can be further in under cover or higher up (Above the frost).
All of these different growing media and mounting methods will work equally well in an indoor situation (Glass house ,Conservatory) with a dead branch as your epiphyte tree but make sure it is a branch from a tree that does not rot quickly. If you are growing only tillandsias then you do not need any growing media at all simply tie the plant to the branch with fishing line. Once the tillandsia has grown roots attached to the branch remove the fishing line. Thoroughly wet tillandsias once a week in hot weather Tillandsias
Most important of all be adventurous and enjoy your gardening . Who says you can't grow a particular plant up a tree or on a pole or rock ?