From what I've read lavender needs a lot of drainage to thrive. Quoting from my favorite gardening book, "Rots out with too much rain and humidity; not tolerant of clay soils". Also . . ."Planting along stone or brick paths laid in sand is an ideal place for lavender".
If you are trying to plant it in soil that won't drain quickly, I'd suggest adding sand to the mix. The only fear of doing that in a planter is the sand washing out the drainage hole.
I myself am going to plant lavender this summer when the rains have stopped. The are I'm planting it in is primarily sand mixed with some some clay soil and leaf mold.
Lavender does best in average to poor sandy soil.

H

Lavender isn't easy to grow in the colder climates.

Try planting it in a protected area (for example, near a south wall). Drainage is very important but if the soil doesn't drain well, amending the soil in a pocket where something is planted won't help drainage; it will simply create a "sink" where water pools. If your soil doesn't drain well, you'll have to find a way to get water away from the planting site. If your soil drains ok, I'd leave that part alone. As mentioned, lavender doesn't like rich soil or fertilizer and doesn't like much water so don't fertilize it and rarely water it (if at all in your area -- I water mine here a few times during the summer but we get less than an inch of rain over July and August -- if you get a good soaking every now and then from rain, you might not need to water it at all). I have some lavender in a planter that faces west and gets hot afternoon sun. The planter is full of garden soil and some rotted compost (no sand but it drains well) and the lavender is thriving. But the Northwest is a great climate for lavender -- I never could grow it well when I lived in Indiana.

If you don't have a good spot in your garden, try growing it in a pot. It'll need to be protected in winter -- put it in a garage that stays cool but doesn't get below freezing, for example. Container growing might be your best alternative if you can protect it in winter.

Good luck!

PS -- sorry, just re-read your post and I have a few more comments.

The window boxes should be fine for the summer, if the boxes are large enough that the roots don't bake (although from my post you've probably guessed they seem to like a fair amount of heat -- mine are in a brick planter about 3' high, 2' wide, and 6' long) -- but stay away from fertilzer and don't overwater. I grew some in pots for awhile that were too small and had trouble but as soon as I got them in larger pots (a couple extra inches for the root ball) they responded well. If you have them in the planter with flowers/herbs/etc. that like water/fertilizer, you probably won't have good luck.

They like full sun; I'm wondering if the ones you had planted in the ground were getting enough?

Think Mediterranean climate (hot, dry, sunny summers) and try to find a spot where you can duplicate that.

I included a link that might help. You can always google "growing lavender" or something similar and see what other sites say. And don't hesitate to contact your local master gardeners -- sometimes they know exactly what's needed in your own area to have success.

Again, good luck!

Here is a link that might be useful: growing lavender

Lavender is evil ;-)

I am in zone 5 Wisconsin and have had the same plants for 3 years now. They don't grow. Fortunately I got decent sized plants to begin with, but they don't grow beyond the size I bought them. They survive, but don't thrive. I am thinking of pulling them up and composting them to make room for something a bit less fussy.

I grow mine in a bed that has clay subsoil, but sandy loam topsoil and lots of compost. The bed overall is dry, not mulched and I don't allow weeds and my sedums love it there so I don't think the soil is staying too wet or has too much fertility. Because my sedums don't require much in the way of water or fert, the lavender goesn't get much of either either.

I think if I were to try and grow these again I would put them in a container and never touch them. Maybe they would like that. In a container though they would be unlikley to survive the winter. Unless maybe I took them inside, but I am too mad at lavender to consider that at this point. ;-)

Lavender hates me too! That's why I grow nepeta instead. It looks like lavender, is long blooming, very hardy and is easily divided. I tried growing lavender one year and they all died! Now that I grow nepeta, including 'Walker's Low' and 'Six Hills Giant', I'm never going back!

I Love lavender! I have lots of white(snow)mulch in the winter unlike most of you. Alot of people have the early spring freezes and thaws that kills it. I mound soil around the woody base every year and cut it back lightly in the spring until it fills out. I grew mine from seed and have had it over ten years. it does get old in the center and thats why I mound.
There is no way Id be without it! How else would I make lavender wands.
Boop

I started my lavender from seed (Munstead--which is supposed to hardy in Zone 5). My first plants are two years old now and thriving. I have them planted in a bed which borders both the street and my driveway. It is poor soil, somewhat rocky, and on the clay-side. I have amended it with some compost. The area does drain well. I also remember reading an article in my local paper about a commercial lavender grower in my area. She said that drainage is key to growing lavender here(grower was located in Topeka, KS). She grows several French and Spanish strains that are not supposed to be hardy here successfully.

has anyone tried a gravel mulch (coarse, not pea gravel) for those in wet areas? i never have and as long as they get sun, mine have been here??? 6 years? anyway, a xeriscaping company gives a suggestion to those of us east of the mississippi, to try such a mulch to protect against crown rot.

I have found the lavender that I grew from seed [ie winter sowed] did very well --mine seem to thrive --but I think the poorer soil the beter and I kind of just neglect them and they grow like the diggins .......I do have some growing with some of my roses which amzingly enough are huge [even though the conditions they like is 100% diffrent from Roses] I Have no prob with Lavender but can kill house plants --does that figure ?????????
Britmum

I was able to get them growing from seed and they did not bloom the first year and died the next spring.

I am trying again in a raised bed which may help the wet feet intolerance issue. If it doesn't work out this time I will move on.
Karen

My lavender is doing well at my house. It is on the south side of the house in some topsoil only about 6 inches thick on top of several feet of gravel (it is near the foundation and the gravel was put in for drainage.) So it has good drainage for sure, and a sheltered location, and gets a lot of sun. It is in generic topsoil that the basement fixing people put in.

My lavender has spread noticeably, and put down new roots to form another plant via the branch in the dirt method (okay I helped a little.) It is just some kind I bought at a grocery store for $2 in a little pot.

I did grow it previously at my last house also in zone 5. It grew okay a few years and then died one winter. That was in really good dirt also near my house on the south side.

Marcia