Yellowing needles

[Phishstix]

Hey everyone,
Curious to get some thoughts on what’s going on here. I have two trees that have began yellowing and I’m not 100% sure what’s going on.
Pic A) Mugo Pine. Lots of wet weather as of later and the tips of this mugo are beginning to brown. This particular tree is in a 5gal pot with a built in reservoir. I’m curious if this is due to too much moisture? It’s currently nearly dry after yet another deluge of water over the past two days.

Pic B) Siberian Larch. These needles have begun to yellow over the last week. 5 gal pot again, but much better drainage. I’m leaning towards sun damage as I think it’s a little to early to begin the fall colours and there is healthy new green growth still coming in.

Thanks for any info/ tips to help get these trees on the road to recovery before winter strikes.

 

[19Mateo83]

Welcome to bonsai nut!

 

[Shibui]

It is difficult to tell what's affecting the trees without knowing a lot more about the pots, potting soil, recent weather and your care.
Dehydration can cause yellow needles and in summer I usually lean toward lack of water as the most likely cause. I find that trees that have not been repotted for some time are at even higher risk as the soil spaces gradually fill with roots leaving no spaces for air and water. Even with good watering less and less water can soak into the root zone.
BUT too much water can cause fungal 'root rot' which means the tree has fewer active roots so cannot take up enough water so it essentially dehydrates even though the soil is wet.
Your mention of lots of wet weather and built in reservoir ring some alarm bells as far as root rot.

Is it possible to slip the trees out of the pots and assess the soil and roots? Pot shape and size and how long since last repot will affect how feasible this is to do. Usually the roots will hold the soil together enough to slide the entire root ball out, check and back into the pot with minimal root disturbance. You are looking to see if the soil is dry down deep or if it is very wet with few white root tips.

 

[Phishstix]

Welcome to bonsai nut!
View attachment 502125

Thanks you so much! Long time creeper of the site!

 

[Phishstix]

It is difficult to tell what's affecting the trees without knowing a lot more about the pots, potting soil, recent weather and your care.
Dehydration can cause yellow needles and in summer I usually lean toward lack of water as the most likely cause. I find that trees that have not been repotted for some time are at even higher risk as the soil spaces gradually fill with roots leaving no spaces for air and water. Even with good watering less and less water can soak into the root zone.
BUT too much water can cause fungal 'root rot' which means the tree has fewer active roots so cannot take up enough water so it essentially dehydrates even though the soil is wet.
Your mention of lots of wet weather and built in reservoir ring some alarm bells as far as root rot.

Is it possible to slip the trees out of the pots and assess the soil and roots? Pot shape and size and how long since last repot will affect how feasible this is to do. Usually the roots will hold the soil together enough to slide the entire root ball out, check and back into the pot with minimal root disturbance. You are looking to see if the soil is dry down deep or if it is very wet with few white root tips.

The mugo is in a pot that is about 12” across and 12” deep. I brought it home and moved it from the nursery pot (8”) into this pot. I added promix and worm castings to make up for the difference in size and have let it sit. I was watering every other day ( soak water the others and re-soak) and fertilizing once every 10 days to 2 weeks. Weather has been steadily in the 20c - 30c range for most of the spring summer.

Im inclined to think it may be root rot as well. We had afternoon heavy rain daily for a solid week to 10 days. I would tip the pot to empty the reservoir daily to avoid wet feet. I’m currently letting it dry out 2” or so down to ensure it has a chance to breathe after a potential drowning. I’m not able to pull the tree to view it as the added soil isn’t compact enough and think it would be a little more of a disturbance than not.

The Siberian larch I was able to slip out and check, soil seems moist throughout and I see white root tips. I’ve moved it to an area with my Japanese maples where it gets late afternoon and evening sun rather than full intensity afternoon sun in hopes it was just over exposed. It’s still in its nursery pot and gets watered until water runs through the bottom as it starts to dry off in the top 1”.

 

[Shibui]

Worm castings rings alarm bells as they do tend to be very moisture retentive and very fine particles so reduces aeration.
Moving from 8" to 12" pot is often known as 'slip potting'. That can be another trap. If the roots are not teased out of the old root ball and it is planted into significantly different soil mix roots will often fail to move to the new soil. In addition water may fail to move between the different soil types so we sometimes find that the inner soil where the roots are gets drier even when the pot is watered regularly because all the water runs through the new outer soil. Conversely inner soil sometimes stays wet because the water will not move to the new soil and drain out. New outer soil can appear dry so we water more and inner root ball gets progressively wetter and wetter until trouble begins.

These are only possibilities as I cannot see and feel your pots. If any sounds like possible then check and do something but if it is way off then please choose to dismiss.
White root tips on the larch is good news. Hopefully it is just sunburn and the tree can recover in part shade.