good question, I wondered something similar about how to sterilize various mixes. I have no answers, but I'd like to know as well...

In my estimation, the only convincing case to be made for reusing container
soils is one of economics, and you'll never find me argue against making that
decision. If you can't afford, you can't afford it. That said and setting economics
aside, you might decide to reuse soil for reasons other than economical.
Perhaps the effort involved with acquiring (or making your own) soil is
something you might not wish to go through or be bothered with.

In any case, it would be difficult to show that soils in a more advanced
state of structural collapse can somehow be preferred to a soil that can be
counted on to maintain its structure for the entire growth cycle. So, if the
economic aspect is set aside, at some point you must decide that "my used
soil is good enough" and that you're willing to accept whatever the
results of that decision are.

All soils are not created equal. The soils I grow in are usually pine bark
based & collapse structurally at a much slower rate that peat based soils,
yet I usually choose to turn them into the garden or give them over to a
compost pile where they serve a better purpose than as a container soil after a
year of service. Some plantings (like woody materials and some perennials) do
pretty well the second year in the same bark-based soil, and with careful
watering, I'm usually able to get them through a third year w/o root issues.

Watering habits are an extremely important part of container gardening. Well
structured soils that drain well are much more forgiving and certainly favor
success on the part of the more inexperienced gardeners. As soils age, water
retention increases and growing becomes increasingly difficult. If your (any
one's) excellence in watering skills allows you to grow in an aging medium, or if
your decision that "good enough" is good enough for you, then it's
(your decision) is good enough for me, too.

The phrases "it works for me" or "I've done it this way for
years w/o problems" is often offered up as good reason to continue the
status quo, but there's not much substance there.

I'm being called away now, but I'll leave with something I offered in reply
on a recent thread:
"... First, plants really aren't particular about what soil is made of. As
long as you're willing to stand over your plant & water every 10 minutes,
you can grow most plants perfectly well in a bucket of marbles. Mix a little of
the proper fertilizers in the water & you're good to go. The plant has all
it needs - water, nutrients, air in the root zone, and something to hold it in
place. So, if we can grow in marbles, how can a soil fail?

Our growing skills fail us more often than our soils fail. We often lack the
experience or knowledge to recognize the shortcomings of our soils and to
adjust for them. The lower our experience/knowledge levels are, the more nearly
perfect should be the soils we grow in, but this is a catch 22 situation
because hidden in the inexperience is the inability to even recognize
differences between good and bad soil(s).

Container soils fail when their structure fails. When we select soils with
components that break down quickly or that are so small they find their way
into and clog macro-pores, we begin our growing attempts under a handicap. I
see anecdotes about reusing soils, even recommendations to do it all over these
forums. I don't argue with the practice, but I (very) rarely do it, even when
growing flowery annuals, meant only for a single season.

Soils don't break down at an even rate. If you assign a soil a life of two
years and imagine that the soil goes from perfect to unusable in that time,
it's likely it would be fine for the first year, lose about 25% of its
suitability in the first half of the second year, and lose the other 75% in the
last half of the second year. This is an approximation & is only meant to
illustrate the exponential rate at which soils collapse. Soils that are
suitable for only a growing season show a similar rate of decline, but at an
accelerated rate. When a used soil is mixed with fresh soil after a growing
season, the old soil particles are in or about to begin a period of accelerated
decay. I choose to turn them into the garden or they find their way to a
compost pile.

Unless the reasons are economical, I find it difficult to imagine why anyone
would add garden soils to container soils. It destroys aeration and usually
causes soils to retain too much water for too long. Sand (unless approaching
the size of BB's), has the same effect. I don't use compost in soils because of
the negative effect on aeration/drainage. The small amount of micro-nutrients
provided by compost can be more efficiently added, organically or
inorganically, via other vehicles.

To boil this all down, a container medium fails when the inverse
relationship between aeration/drainage goes awry. When aeration is reduced,
soggy soil is the result, and trouble is in the making.

Al

The main reason i always use compost is to prevent root rot. This practice allows you to use soils for as long as 7 years. I have some as old as 12 years. The plant grows like a weed. It's almost in pure compost now. Using compost gives amazing results. I was sold long ago. I usually change my soil every 3 years, but noticed some plants do fine long term. My bigger tropical trees are a pain to change soils, I do them every 7 years. Most are 35 years old. A couple are 39 years old.

If you have a problem with the compost, a alternative would be to use compost
tea. You get the benefits of compost, without getting the compaction. of actual
compost. Why I think it is worth it is the large number of beneficial bacteria
and fungi, they make for a healthy environment. It can't hurt either. I usually
never have a problem with root rot, but some might benefit. I never get root
rot, but I do use compost. It also works really well to stop damping off in
seedlings. Here is a study about cucumbers and root rot. Many other studies around, but
it shows how many beneficials are in compost. At least consider it for your garden!

http://www.hindawi.com/journals/jmy/2013/324570/

From this study, I thought this chart was interesting they used a number of different composts, and every single one reduced the incidence of root rot compared to the control with no compost. IMHO it's not just root rot, but a number of problems can be prevented with the inclusion of compost. Also noted in the study is extracts didn't fair as well as using compost itself. (not shown in graph).

http://www.hindawi.com/journals/jmy/2013/324570/tab8/

In general I would suggest not keeping soils beyond three years. As far as
the OP and white flies, not sure if it is cool to reuse? I never dealt with
them. If they form larvae in the soil, it might be a problem. You could maybe
buy a few bottles of hydrogen peroxide and dilute 50-50 and drench soil before
you put plants in. The stuff is cheap! This should kill any eggs or larvae. Works well with fungus
gnats too.

The reason many growers here don't like peat-based soils, even when fresh from the bag, is their excess water retention. That problem doesn't get better after a year of additional particulate breakdown, which is to say it's not 'fixable' by adding more fine material like compost which only continues to support or increase excess water retention.

I fully realize we need to look at issues from more than one perspective, but I argue that from the plant's perspective, it's not a good idea to reuse peat-based soils except perhaps as a small fraction of another soil we're making, and trying to add compost to a soil that already holds too much water in the hope it will make the soil salvable is probably a wasted effort. If the needs of the grower trump the needs of the plant, so be it, but that has no bearing on what offers greater potential for the plant.

Al

But does anyone know, if the op's whiteflies will come back? That was the question, after all.

Hard to say .......... They over-winter on the leaves of host plants and generally can't survive outdoors in freezing conditions, so it's unlikely the medium itself would be a vector for recurrent infestations.

Al

Yes, thank you! I appreciate the info on degradation of peat-based mixes and I am thoroughly aware of those issues (and I think I've read that exact post before from Al?), but my question is about whiteflies and whether they can survive in the soil over winter. Anyone dealt with that before?

I'm guessing you were probably typing your question about the time I was finishing my last offering, which has some info that addresses the question directly.

Lol - I have a couple of hundred files with replys to FAQs that I copy/paste in appropriate places - saves a ton of time and mental gymnastics. That's why you often see the same old stuff over & over. If I'd known you've read it, I would have spared you. (Not said with any sarcasm - or meant to come off as biting).

Al