The rind at the soil spot should toughen and resist denting with a fingernail when the melon is ripe. Finally, for those with an ear for music or who can easily distinguish sounds, the ripe melon will have a dull thud when thumped, while the unripe melon will have a tighter, ringing or hollow sound. This publication gives guidelines for determining when to harvest 40 common garden vegetables, including watermelons, cantaloupes, and pumpkins. Our recent rains have provided the perfect conditions and the goatheads are growing where they haven't been seen growing so densely for several years. You cannot use a preemergent herbicide to control weeds which have already germinated.
Preemergent herbicides are to prevent the seeds from sprouting.
Once the seeds have germinated you will need to use mechanical control, hoeing or pulling, or a postemergence herbicide labeled for controlling your target weeds. It is important to know that there is no herbicide labeled for goathead weeds. The name you will see listed on the herbicide container is puncture vine. Look for a product labeled for post emergence control of puncture vine weeds. Be sure to read the instructions carefully and follow the directions.