How to tell Juniper ready to repot

[777thSunRay]

I have a Juniper Procumbens which I bought as pre-bonsai, and have been growing about a year and a half, in a 1 gallon pot. I pruned it once, though a bit to excess, which is why only once. It has grown pretty bug, and I am wondering if the roots have filled the pot. How can I check? I am thinking to use a wood skewer to stick into the soil and feel around for heavy root density. Is this a good method, and are there any others, possibly better?

By the way, I am wondering, is Procumbens a species, or a specification of a down-sweeping training (or is it a species which tends to sweep down, and hence the name?)

The ends are growing down. And I took too much off of the top. With this variety, I am thinking I will train a lead or two to sweep upward at a new angle. Is this a good practice for this variety?

By the way, I will likely repot to an air pot, something like the attached image. Is it okay to use a mix with some soil with this, or if not, what about vermiculite as one element?

 

[Shibui]

Juniper procumbens is a species. The name means 'laying down' which describes how it normally grows - flat along the ground. We can train the trunk to stand up but new growth does not have upright habit so tends to grow downward again. You can keep training those branches up if you wish or work with the natural habit to grow down.
Whether it is appropriate for any individual tree depends on the current shape of the trunk and branches. We want our bonsai to look good and look natural. Sometimes allowing the tree to grow as it wants does not give those attributes so we need to intervene. Can't really give any meaningful advice without seeing the actual tree.

As for roots, I usually just press down on the soil. If I can compress the soil a bit it means the soil is not yet full of roots and does not need repotting. If the soil is very firm that indicates that the soil is full of roots and it's time to repot.
You can elect to repot before the soil is pot bound. Junipers don't like too frequent root disturbance but if it has been over a year the tree should cope with some roots being cut again.
A bit earlier in spring would have been more appropriate and safer to repot if you plan to root prune but if the plan is just to move the root ball into the air pot without too much root reduction that can be done any time.
Use whatever soil you normally use to fill the air pots. Because of the extra drainage and evaporation from these pots you can get away with less well drained soils in these. The downside is that any new, finer particle soil will probably need to be removed before the tree goes back into a smaller bonsai pot.
Vermiculite is usually very fine particles and breaks up even more as roots grow so not really used for bonsai potting soil. Perlite is a possible component but it's quite light and pots have a tendency to fall over or blow away when perlite is a large part of the soil mix.

 

[777thSunRay]

Juniper procumbens is a species. The name means 'laying down' which describes how it normally grows - flat along the ground. We can train the trunk to stand up but new growth does not have upright habit so tends to grow downward again. You can keep training those branches up if you wish or work with the natural habit to grow down.
Whether it is appropriate for any individual tree depends on the current shape of the trunk and branches. We want our bonsai to look good and look natural. Sometimes allowing the tree to grow as it wants does not give those attributes so we need to intervene. Can't really give any meaningful advice without seeing the actual tree.

As for roots, I usually just press down on the soil. If I can compress the soil a bit it means the soil is not yet full of roots and does not need repotting. If the soil is very firm that indicates that the soil is full of roots and it's time to repot.
You can elect to repot before the soil is pot bound. Junipers don't like too frequent root disturbance but if it has been over a year the tree should cope with some roots being cut again.
A bit earlier in spring would have been more appropriate and safer to repot if you plan to root prune but if the plan is just to move the root ball into the air pot without too much root reduction that can be done any time.
Use whatever soil you normally use to fill the air pots. Because of the extra drainage and evaporation from these pots you can get away with less well drained soils in these. The downside is that any new, finer particle soil will probably need to be removed before the tree goes back into a smaller bonsai pot.
Vermiculite is usually very fine particles and breaks up even more as roots grow so not really used for bonsai potting soil. Perlite is a possible component but it's quite light and pots have a tendency to fall over or blow away when perlite is a large part of the soil mix.

Thank you for this. Would you suggest a Juniper species which tends to grow upwards naturally? I am considering getting a Chinese Jupiter, because I've read they are the biggest Juniper. Is this true? Though I've read they are indoor species, are they? What is a fast growing upright growing outdoor Juniper I might search for? I tend to prefer bigger Bonsai, though not always!

The soil surface is still somewhat soft, so I'll wait awhile, thanks.

So our spring has been unseasonably cold and overcast, so hopefully it's an okay time to repot??

I am now using Miracle Gro cactus mix with pumice. I am thinking of switching over to pumice + mulch chips + lava rocks (plus a bit of perlite, as you mentioned). Is it okay to switch? Can I add the Miracle Gro I mentioned to a mix like this? I have the impression it's not a good combination, though I'm wondering why. Also, what about sand... I have a sandbag which has ripped open lol

Are there any other substrate components that you might suggest?

Thanks again!

 

[ShimpakuBonsai]

I don't think a 1 gallon pot will be full of roots after 1.5 year.
Maybe you can lift the tree out of the pot and look at the roots and put it back after.

 

[Shibui]

We can train almost any juniper to grow in any direction. Thousands of J. procumbens are trained with an upright trunk every year no problem. The prostrate habit actually helps beginners form good branch pads with less effort so they are very popular for all shapes of bonsai.
Juniper chinensis (Chinese juniper) is a very variable species because they grow in many different places in China, Japan, Korea. Some are large trees but others have adapted to be small bushes on high mountains. We mostly use the smaller types as bonsai. I have some in our garden as landscape plants and they are still only waist high after 15 years so definitely not true that all Chinese junipers are large. They are extremely popular for bonsai, mostly because of the attractive and compact foliage of the types used for bonsai. Very good choice if they are available but quite slow to grow and develop. I have trees I have been training for 15 years and are still not ready to be planted in a bonsai pot (my standards are probably quite a bit higher than most beginners though)
All junipers are outdoor plants. They tend to die slowly when kept indoors. In most of Ca I think they would be outdoors all year round as are most species here.

Check some threads on soil mixes here. Everyone seems to have their own favourite mix but unfortunately everyone also believes that their mix is the only mix that anyone should use. The reality is that many growers manage to grow great bonsai in many different mixes. Everyone needs to work out what works well for them in their back yard in their climate with the time and expertise they have. With all those variables it's no wonder different growers use different mixes.
Many growers use cacti mix with or without added extras. If it works well at your place then keep using it. Making your own mix can have hidden dangers if you don't understand the properties of drainage, water holding capacity, cation exchange, pH, nutrient drawdown and lots more technical stuff. Much safer to use something that someone who does know all that stuff has put together because it should work.
Sand can be a useful addition but sand can be all sorts of sizes and all sorts of composition. Sand from sandbags tends to be quite fine and is NOT a good addition. Bonsai mix seems to work better with larger particle sizes which allows more space for water, air and roots. Coarse sand can be useful. Beach sand may contain salt which is not good for most plants. Limestone sand may be alkaline which can also harm plants. Definitely know your sand before deciding to add any to potting soil.
Pumice, mulch chips and lava is used by bonsai growers so could be OK for you but it will be different and will require different watering regime, etc. Adding cacti mix may be good but if the cacti particles are much smaller than the others they can filter down to the bottom and slow water drainage which causes new problems. Best to have relatively uniform particle size in any mix.