There are times when the trees you need are not available. Maybe you just need foliage images. Or perhaps your project will have a mix of tree images and tree models. There are several solutions available depending on your budget and how much time you have to create the models. The following applications are not an exhaustive list. They are the ones that I've used and have found to be the most useful for the widest range of applications.

Xfrog: If you don't have the time but have the money

Greenworks' Xfrog is a versatile, though not intuitive, program. It does not model trees using a tree morphology paradigm. But it does have XfrogPlants, an extensive library of ready-to-render tree models available for purchase. If they have the models and you have the money, all you have to do is open the models in Xfrog and render your foliage. If you are not an Xfrog modeler and are on a project deadline, Xfrog and XfrogPlants will get the job done. See my Amazon Rain Forest feature to see a VNS scene rendered using XfrogPlants.

Xfrog 3.5 user interface

XfrogPlants eucalyptus rendered with shadows in LightWave

OnyxTREE: If you have the time and want a foliage-specific interface

The Onyx suite of parametric tree modelers is the most biologically faithful I've worked with. My older Tree Pro 6 suite includes bamboo, broadleaf, conifer, and palm applications. The current suite is OnyxGARDEN and also includes grass and flower modelers. Each application is designed around the tree type with controls for trunk, bough, branch, twig, and leaf. Export options are plentiful and useful. Many parameter files are included for common trees but expect to spend time modifying them for your use. While some leaf images are included, you'll have to texture the bark yourself. If you just want images, that means you'll need to use a 3D application to texture and render the images. Although both Xfrog and Onyx can export models as images, I use LightWave for shadow control and higher quality renders. You can also use VNS to render images from models as described in this tutorial.

OnyxTREE Broadleaf user interface

Plant Studio

I have not used Plant Studio for several years, but it's powerful if you need herbaceous plants and parameter-driven plant growth. And it's still free!