Pathways should flow like a meandering stream not a straight road. As to trees, they are fine as long as they are not placed in a direct line with any entrances, especially the main entrance.
Shrubbery is okay as long as it is kept low lying as not to create a mountain formation. Trees and shrubbery can be used to block harsh Qi . Grass does not pose any problems. Strive for beauty.
Flowers and other colorful plants can be used as long as they fit within the environment and follow some of the basic rules of the 5 elements and theories of environmental feng shui .
"Wood produces fire, fire produces earth, earth produces metal, metal produces water, water produces wood." Wood is green, fire is red, earth is tans or yellows, metal is white or gold, water is blue or black. Avoid using them in a destructive combination such that "wood uproots earth, earth blocks water, water douses fire, fire melts metal, and metal chops wood".
Consider the direction the building "sits." For example if the building faces to the North and sits to the South, than this is a Li building (Li is the Fire trigram). This house would be supported by a lot of red flowers, but would not benefit from blue or black colors. A house that sits to the west and faces east (Tui is a metal trigram) would benefit from white and gold flowers. Reds and purples should be avoided. This can also apply to the color of the house and the trim as well. These are all general rules and while their effect can be minimal on the house itself, following the rules of balance can support the house and its occupants.
The two most important areas in landscape feng shui are that need to be looked at carefully are rocks and water. Since they are both part of the five elements and are used to remedy certain situations in the greater environment, they need to be placed very carefully.
It takes a qualified practitioner to determine if the water will have a positive or negative effect. If placed incorrectly the effects can be devastating to the residents. If you are striving for the effect of the water element. Ponds and pools can be used if placed under the guidance of a trained Feng Shui practitioner.
The same can be said of rocks, especially large decorative rocks. Because these are used in certain situations to correct for a problem, they need to be placed under the guidance of a Feng Shui practitioner. Placed haphazardly, they could have an adverse effect on the prosperity of the building This takes a qualified Feng Shui practitioner to determine if the placement of these elements is needed or not. Each building is unique and needs to be looked at on a case by case basis.
To summarize, plants and other "wood" elements are fine anywhere as long as they do not block the Qi of the building and are used in balance. Colors can be used to assist the home as long as they fall within the rules of the five elements. Water and Rocks need to be placed under the advice of a trained practitioner to avoid adversely effecting the people or the prosperity. When placed correctly, they can have a very supportive and prosperous effect on the residents. All in all, the goal of Feng Shui is to create a comfortable and beautiful environment for the people. And to support the health, happiness and prosperity of the residents.