Now that the rock castings have been painted to your satisfaction, you still have wide expanses of barren white hills. Covering them is done by using something called ground foam. The most common brand of these foams is Woodland Scenics, but you may encounter other brands. The ground foam is available in coarse and medium textures, and in several different colors. You may also encounter a product which Woodland Scenics calls Foliage. Although similar to ground cover materials, it is used to represent tree foliage rather than ground cover.
First, indoor household latex paint is applied to areas that will be textured, taking care not to spill any onto the detailed rock castings. This paint serves to seal the plaster of Paris (which tends to produce a white dust over time) and to act as an adhesive for the first layer of the ground foam. The foam is gently shaken from the bag onto the wet latex paint. Areas where the foam does not adhere well, such as steep hillsides, can be whisked by blowing the ground foam from the palm of your hand onto the steep surfaces. Later applications of different ground foams will serve to feather the edges of the rock castings, making them look even more real. Incidentally, if you have built a number of Z scale structures, you probably have a supply of small packages of colored ground foam left over from various kits. This foam, used to represent window boxes and beds of flowers in front of other structures, can be used here to represent wildflowers. Apply this type of ground foam sparingly, for a little goes a long way.
After the initial layer of ground foam is applied, you can follow up with additional layers. This can serve to enhance scenery detail, to cover gaps and open spots, and to cover mistakes. These layers are applied over the previous layer simply by shaking the material out of the bag. Once the new layer is in place, fill your trusty spray bottle with water and a few drops of liquid detergent such as Ivory to make a wetting agent. Dampen the newly applied layer of ground foam with the wetting agent. Then, apply a solution of the matte medium with an eye dropper, or a turkey baster if the area is large. This solution should be about 20 percent matte medium, 5 percent detergent, and 75 percent water (although you may have to adjust this to suit your needs). Without the wetting agent, the matte medium solution would tend to form a ball of solution on top of the foam instead of penetrating into it. Once dry, the foam looks loose, yet it is held firmly in place.
It is tempting, as you gaze at the wide expanses of pristine plaster of Paris hills, to think about doing a snow scene. This isn't a bad idea, and Faller makes a kit that features a mix similar to plaster of Paris that can be applied to rooftops and equipment. A material like diamond dust is applied on top of the wet snow mixture, to give it that sparkling look of new snow. Please note that snow does not always accumulate on every surface; vertical walls and rock cuts will not have snow on them unless it has been windblown. The Faller kit also includes plastic icicles, but they are really too large for Z scale. You could try silicone resin as a substitute.
A drawback to snow scenes is that they show dirt and dust much more quickly than other layouts, so gentle vacuuming will be necessary. In spite of this shortcoming, you may well be pleased with a snow scene.
As much as I would like to believe otherwise, roads and highways are a necessary part of railroad life. Roads can be represented in several ways. You can apply plaster of Paris, much in the manner of concrete, directly on to the hill forms and the layout support. Poster board and shirt cardboard can also be used to represent roads. In any case, they will need to be painted. When wet, concrete is a medium gray color, but after drying, it seems to become more buff-colored. (I like a color made by Testors, called Model Master Desert Sand.) The expansion joints of concrete can be drawn in with a pencil and straight edge. Noch makes several roadway materials that are lithographed onto self-adhesive tape. This tape is applied to the scenery before the ground cover, which serves to feather out the edge of the roadway in a natural fashion.
Trees can be represented in a variety of ways. In a scale as small as Z, the quickest way seems to be to use clumps of lichen. Lichen is a natural moss, sometimes called reindeer moss. The commercially packaged product is treated with dyes and glycerin to keep it supple. Lichen comes from several manufacturers, but the best seems to be made by LaBelle. The stuff can be expensive, and on top of that, is often hard to get. In addition, not all the material that comes in the bag is usable. You will need to clean the lichen by removing sticks, stems, and pine needles. Afterwards, you will find that not all the pieces of lichen suit you. Sorting the lichen is a test of my self-discipline, since I really want to use all that I have paid for. Unfortunately, even after trimming, some pieces simply don't look realistic.
There are tree kits available, notably from Woodland Scenics. Tree kits are not marketed by scale, usually, but by size (expressed in inches of height). A tree kit contains metal castings that you bend to shape, spray paint, and then cover the branches with a mesh coated with ground foam (the foliage material I referred to earlier). If you use these kits, you need to exercise some caution, since a four-inch tall tree casting will be incredibly huge in Z scale, over 73 scale feet in height.
There are some ready-made trees available, but some of these are either too big or unrealistic. Again, Noch has several items specifically for Z. You could try using twigs found in the yard or park, with bits of lichen for the foliage. Do some experimentation until you find an approach that you like, but be cautious about the overall size of the trees; an oversized tree will make the Z scale trains look toy like.
You may have noticed that I have been carefully avoiding a certain topic, water. It is possible to represent water, sometimes very convincingly, but realistic-looking water needs thought and work. And, no, real water does not work well.
One of the easier methods uses a product from Faller called a lake construction kit. The kit is a sheet of rippled translucent plastic that allows colors and general shapes to be seen, although not clearly. The plastic sheet, which measures 12" x 24", can be laid directly on the layout sheet. However, I have found it easier to cut out an opening in the layout and then install the sheet in the opening.
The most effective technique begins with making a pattern. Using paper or poster board, cut out the shape you want and then use this pattern to mark the layout. For a simple pond, the shape would be an irregular oval. Once marked, the opening for the pond can then be cut out of the layout. Using the same pattern, mark the water sheet from the kit and cut out the shape of the water, leaving a margin of about an inch. This eliminates the possibility of gaps along the shoreline. The water sheet can then be trimmed as necessary for a proper fit. You also can apply the water sheet from the bottom side of the layout, although you may find the shore line to be awfully steep. In any case, a little cutting and fitting may be necessary, but further application of ground foam should cover any raw edges or unplanned gaps.
Another relatively easy way to represent water is to use gloss medium. This material, cousin to matte medium, is applied to produce a glossy surface. The stream or pond is created by pouring plaster of Paris onto previous layers of paper towels, and then smoothing and shaping it by hand. After this area is dry, paint it with flat model paints or acrylics. Use darker greens to represent deeper areas and lighter greens and browns to represent more shallow areas. After this is dry, apply two or three coats of the gloss medium to produce the shiny effect of water. For flowing streams, you can dry brush, very lightly, thin streaks of gloss white or silver to represent the bubbles that form in water as it rushes over the stones of the stream bed. Once the gloss medium and paints are completely dry, complete the edges of the stream or pond with ground foam, remembering that vegetation so close to water is lush and green.
There are other methods of representing water that involve using epoxies and casting resins. I have not had much experience with these approaches. On one occasion, I used casting resin unsuccessfully, probably because I failed to read the directions thoroughly. Not only was the result disastrous, but the chemical reaction produced a particularly strong chemical odor (which may prove useful for you if you are trying to break your apartment lease or divorce your spouse). Your hobby dealer may be able to help you with other water techniques.
You are just about finished. The ground foam covering is just right, the lichen has been applied to the layout, the tunnel portals and retaining walls are in place. All that is missing is the ballast. Ballast is used on real railroads to hold the track in place, while allowing rain water to drain away from the right of way. Ballast is usually gravel and because the railroad uses so much, it is usually obtained from a quarry close to the railroad. This is the reason that railroad ballast differs from one part of the country to another. In Georgia, much of the ballast is granite; in other areas, it is made of other locally available rock. Also, railroads that are experiencing financial difficulties may use cinders left over from the combustion of the coal by their locomotives for their ballast. It doesn't drain as well as rock, but there usually is lots of it and the price is right.
You can use different colors of ballast to emphasize the existence of different railroad companies on your layout. Each railroad may be ordering ballast from quarries that are shippers on their own line. You also can use cinder ballast on branch lines or on industrial spurs.
From a model standpoint, ballast serves to tie everything together. At least two manufacturers produce ballast appropriate for Z scale. Woodland Scenics produces a fine ballast, and Highball Products produces a Z ballast. It is most important that the ballast particles used be not too large. The ballast is applied to the track area, and is used to cover the raw edges of the scenery that were created by the earlier removal of masking tape.
I prefer to apply the ballast dry, using a paper cup to roughly dispense the ballast onto the area between and around the crossties. Use a brush to sweep the ballast into its final location. Take extreme care to keep the ballast particles away from the points and frogs of turnouts so that the wheels of the train can pass unimpeded.
Once the ballast has been placed in position, dampen it with a very fine mist of the wetting agent you used to apply the layers of ground foam. Then, thin the matte medium with water and apply it with an eyedropper to hold the ballast in place. Again, be especially careful not to dampen the points and frogs of the turnouts with either the wetting agent or the matte medium.
When everything has dried thoroughly, you will need to clean the rails to remove any medium which may have accumulated, giving special attention to the points and frog. If the medium has got into the points, it often can be peeled off, since it dries not only flat, but flexible. A Bright Boy or other track cleaning pad will work, but your fingernail may do the job just as well and not scar the rails of the track. As mentioned in Maintenance, you don't want to use anything too abrasive.
If you need more information on scenery techniques, or are curious about other approaches, I recommend a couple of books.
Scenery, by the late Bill McClanahan, is an excellent book that deals with the whys and wherefores of scenery. It is really an abbreviated model railroad geology course that also provides information about making scenery. Many of the techniques are dated, but the geology study is worth the cost of the book alone.
How to Build Realistic Model Railroad Scenery, by David Frary, is a more recent book, and as such, deals with modern model railroad scenery techniques. The materials used are water soluble, so everything cleans up easily.
The thing to remember about scenery work is that it is quite forgiving, and mistakes can usually be corrected with other scenery materials. It is also an opportunity for some hobby fellowship, where you can get together with others to work for your common good. There is nothing like shared experiences, especially when your hands are covered with plaster of Paris and the telephone is ringing.