Check the roots. If you see that the roots are well developed and you're holding a plant that grew in this pot for some time, go ahead, plant it. If you notice that roots still not well developed keep it in the pot till fall. In my experience - the bigger the root ball the better outcome. Keeping in the pots might be tricky in the hot summer as they getting dry way too fast and the plants will be slow to grow. I killed several this way. Now I plant small sized plants in 3 - 5 gal pots so they don't dry too fast, keep them there for a year or so and then plant in the ground.
