Past participle/estar/ser
 

Are any of the following correct?

The roses are planted in front of the fence.
Las rosas son plantadas delante de la cerca.
Las rosas están plantadas delante de la cerca.

The roses are planted in front.
Las rosas son plantadas por delante.
Las rosas están plantadas por delante.

Yes. (In my non grammar way) :-)

Consider this.

The roses are planted in front of the fence.

Would this mean in English that:

The roses have to be planted in front of the fence?

The roses are actually in the spot defined by the front of the fence?

The forms are correct, from my usage point of view. :)

Okay - so they're all correct. But what is the nuance in difference between the "ser" and the "estar" sentences?

"Están plantadas" would be correct to say where they have been planted.

"Son plantadas" is passive voice; in this case it would mean they are being planted at that moment.

Or you'd need a complement (and it's still present tense):
"Las rosas son plantadas por el jardinero"
"The roses are planted by the gardener"

Psst - don't tell Hernan, but after I have done some more work with "para" y "por", I'm going to work on "ser" y "estar". :)

Special thanks to laepelba for asking about ser/estar difference. That is what I really wanted to know, but I forgot to ask.
Does Se plantan las rosas have the same meaning as Las rosas son plantadas?

Yes! The latter is the passive voice, as Angelica said. Be careful. The passive voice is not as common in spoken Spanish as it is in English and it tends to get overrused by English speakers.
Using the 'passive se' (se plantan) construct is much more popular in everyday speech. Using a reflexive verb, followed by the subject, doesn't immediately make sense to the beginner, but it sounds very natural to the Spanish speaker.
For example:
Se habla español (literally, Spanish speaks itself)
= Spanish is spoken (here)
Se venden flores (literally, flowers sell themselves)
= Flowers are sold (here)

Aha! I am gone for a couple of days and this is what I get. :-)

I saw you are more proficient at para/por.

Or maybe I'll work on pronouns.......... :D

Do you eat those? :D