It is well-established that pauses and final lengthening are phonetic correlates of prosodic boundaries. Another line of research has established that speech planning is known to be reflected in speech production; for example, longer pause durations are observed as the amount of planning needed for the upcoming utterance increases. Combining these two lines of research, the present study examined how speech planning and prosodic structure interact in speech production, focusing on temporal properties. Two questions were addressed in an electromagnetic articulography study of American English. The first question examined the effect of an increase in planning load on gestural and pause duration at prosodic boundaries. The results showed that only pause duration, but not gestural duration, is longer when more planning is required for the upcoming material, implying that speakers use only pause duration for additional planning at prosodic boundaries. The second question tests the effect of an increase in planning load at word boundaries. Along with the insertion of a pause, lengthening of gestures before and after the pause was observed, indicating that speakers insert prosodic boundaries for planning. Implications for the role of prosodic boundaries in speech planning and the nature of prosodic representation are discussed.