Ask Mr. Smarty Plants

We did some research and discovered no instance in which eucalyptus, which is native to Australia and therefore out of our range of expertise, poisons soil. We did, however, find this article from the U. S. Fish and Wildlife Service Environmental Containment Program Eucalyptus Trees Used to Clean Up Selenium in California. Since you say you were in California, perhaps this is what you heard about.

So far as we know, neither does any member of the Juniperus genus poison the soil around it. We don't know which members of the genus you have, but the ones native to Colorado are: Juniperus communis (Common juniper), Juniperus communis var. depressa (Common juniper), Juniperus monosperma (Oneseed juniper), Juniperus osteosperma (Utah juniper), Juniperus scopulorum (Rocky mountain juniper), and Juniperus virginiana (Eastern red cedar). You can follow each plant link to our webpage on that particular species of Juniperus, and read the information about its use and care.

The next question is: Are you trying to plant that garden under those junipers? From a previous Mr. Smarty Plants answer:

"As for the Juniperus ashei (Ashe's juniper) on your property, there is some disagreement whether anything will grow under them. Again, you have the heavy shade, and the litter of needles, berries and twigs always on the ground, which will discourage many plants and inhibit seedlings. 

From this article  Biology and ecology of Ashe Juniper by F. E. Smeins and S.D. Fuhlendorf, we extracted this paragraph on the allelopathy of the Ashe juniper:

'There is little evidence that the accumulated litter of Ashe juniper in any way alters the chemical nature of the soil as it relates to growth and development of other plants (Yager 1993). Soil chemical and physical properties are in fact "improved" by the presence of the juniper litter (Marshall 1995). No allelopathic effects have been shown to be produced by the litter. The major impact of the litter seems to be its physical presence and its alteration of hydrologic properties of the area under the canopy. The thick litter layer is a difficult physical medium for seeds of other species to germinate and grow in mainly because moisture either runs off (due to the hydrophobic nature of the litter) or it dries out very quickly after a precipitation event, which prevents seedling growth from reaching the mineral soil (Yager 1993).'

'Junipers create an environment under their canopy.' They do indeed. That environment includes dense shade and a lot of shed material like leaves, berries and bark (aka mulch).  Add to this that the shed material is hydrophobic ( it sheds water rather than absorbs it) and I ask you what could be less conducive to existing plants?"

You need to till the litter from the junipers and oaks into the soil and plant natives appropriate for your area, soil, light, drainage and enjoy. The juniper is considered a nurse plant for many plants that grow below it and bask in the conditions.

Now, here is the catch-did you note we said "plant natives?" If you are trying to grow plants not native to your area of Colorado, you may be blaming the juniper for something that is not its fault.