About now - at least when the frosts have passed - seeing how you're further north! Dig a decent hole and water the bottom well - to encourage the roots to go down, before adding some compost and mixing it into the soil, then add the plant, make sure it gets well watered in, mulch around the top and it should grow brilliantly. There should be no need to water further unless we have a particularly dry spring. Keep an eye on it in the first year - if it starts drooping and it's been dry, give it a really good soaking. If your soil is poor, give it a topdressing of compost each spring and maybe a handful of blood/fish/bone or chicken manure granules. good luck.