You should divide it only if it actually needs it (every 4-5 years). If you have patience, you'll get your monies worth, otherwise you will have bought an expensive plastic pot.

I have this one too,I bought it originally to put in a planter.It has done better in here than out in the garden,where it hasn't spread after 2or3 years.
In the planter I have had to divide it to leave room for other plants.It grows tiny plantlets at its base which are easily pulled gently apart to keep some roots intact.
See my photo's page it is planted with a hebe :)

Thanks Wylieinthea.. and Aster, intending to put it in planter. Aster your one looks very healthy, what compost have you used and do you feed it?

they also produce black berries that will grow happily too. I do that too.

I think it grows better in shade and I divide mine freqently to increase the ground cover. It likes to be kept moist through the Summer to keep growing.

From seed you do not get 100 percent Black plants, about 1/3 may well be ordinary green Ophiopogon.

Mine is just in a compost for pots and hanging baskets,whatever I see at the GC.
It is in full sun but as I water everything else if they need it doesn't suffer,also general fertilizer occasionally.
It seems to like the drier conditions in the planter,the others in shade for half of the day haven't spread.
Sorry Poaannua I don't mean to contradict but it is just my experience of these plants,maybe my shade in Lincs is cooler than yours further south :))

Thanks all for your comments. I shall enjoy seeing under what conditions my plant flourishes.