Spacing Limelight

clairetn(7)December 8, 2009

I am new to this forum and appreciate all of the helpful information I have gathered through reading posts on gardening. Hopefully you can offer me some advice concerning my recent planting of 14 Limelight Hydrangea.

I have a 7 to 8 foot wide space between a retaining wall and concrete patio. All conditions good for growing Limelight. Yesterday we dug 14 very large holes, added compost and peat moss for good loamy soil and 4 inches of mulch.

I am going for a tight hedge (like some of the photos I have seen on the internet). However I believe I made a grave error in the spacing as I put them 4 feet apart. Is this going to cause any harm to the plants? I dread thinking we need to go back and replant all of them. It was hard work!

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Forgot to ask, what would the results be for planting this close in addition to any harm done to them?
Thank you!

    Bookmark   December 8, 2009 at 9:38PM
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That is a bit close - this one can grow to 6-8' x 6-8'. Generally, ideal spacing would be on the lower end of the scale (6'). But it does take pruning well (and still bloom!) so that is an option to keep the plants in check. I would not expect any particular drawbacks from a tighter spacing other than some shading out of lower foliage. And the more rapid spread of any diseases, should that be a concern.

FWIW, there has been a great deal of research done on amending individual planting holes and the recommendation is to avoid this practice whenever possible. Plants will establish faster and develop better root systems if they are planted into indigenous, unamended soils. Plus, amending individual planting holes can create soil interface issues that impact good drainage and water percolation down into the soil levels. If you need to amend, do so over a very large area, like an entire planting bed, rather than individual planting holes.

    Bookmark   December 9, 2009 at 11:38AM
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Gardengal, thank you very much for the information. This is one of those situations where I am bonking myself on the head...I should have known better! The shading out would appy to the crowded parts of the plant touching each other I would assume. I am hoping since I have generous space in front and back of the plants that they will fill out in those spaces. Pruning is no problem as I plan to cut them back during late fall since I like to tidy up for winter. If I see that there are problems in the future because of the tight spacing I will move them at some point. I just can't do it right now as I am limited on time and help at the moment.

Also, thank you for the tip on amending planting holes. I am going to try to work some good compost in between the holes a bit at a time, a practice I do often as I compost my kitchen scraps. I have learned since moving here a few years ago that we have what they call chirt, a hard rocky red clay soil that needs lots of improvement for planting beds.

I am crossing my fingers for success next spring! I fell in love with Limelight after seeing photos of them! My sister has them and people stop to ask her what they are.

Claire : )

    Bookmark   December 9, 2009 at 7:19PM
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"...there has been a great deal of research done on amending individual planting holes and the recommendation is to avoid this practice whenever possible."

Pam, could you refer me to a few academic researches/abstracts on a subject.

    Bookmark   December 11, 2009 at 12:06AM
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George, a lot of the initial studies into this were conducted by Dr. Carl Whitcomb several decades ago. In fact, if you search for data on this subject, you will almost always find references to these studies. Some of these are published online but the bulk of the info can be obtained in his text Establishment and Maintenance of Landscape Plants, published in 1986 and revised in 1991. It is considered a planting 'bible' by many in the horticultural profession :-)

And there are other, somewhat more current studies as well. Some of them are plant-specific but the theory remains intact for any plant type.

The Myth of Soil Amendments

Backfill and Planting Hole Shape

Effect of Backfill Amendments

Clemson University: Tree Planting Fundamentals

Most of the concern focuses around soil interface issues - the different textures of diverse soil materials: the soil surrounding the root mass (containered or B&B stock), the amended soil of the backfill and the indigenous soil. If of sufficiently different texture and porosity, which they almost always are, water does not percolate efficiently through the various textures and you can wind up with either very dry rootballs or those that are sitting in overly saturated soils. There is also evidence to support the notion that highly amended planting holes encourage the root systems to remain within that enriched soil, not spreading out into the surrounding native soils. The result is a circling, girdling root system that retards plant establishment and ultimately resulting in an inferior, underdeveloped root mass. If you expand the amended area to include the entire potential root zone, or in the case of smaller shrubs and plants, an entire planting bed rather than individual planting holes, you overcome much of the soil interface issue by satisfactorily blending the disparate soil textures.

    Bookmark   December 11, 2009 at 12:30PM
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Not amending the soil is not practical in many areas ( heavy clay ,same for sand) I watched a seminar where the speaker showed how to dig the hole to the recommended size for the plant in guestion then used the shovel to really loosen the surrounding soil in the bottom of the hole and sides then threw in a few shovelfuls of ammended soil and reloosened the soil again with the amended soil. Then added amended the plant and more amended soil . The result was a bridge from amended soil threw to the native soil. It makes it easier for the plant to root outward and down ward and keeps it from running into a adrupt change from ammended to nonammented soil. Instead the plant goes from ammended (which most plants are potted in already so its hard to get around even if you wanted to) to ammened cut into native to native soil. For those of us with heavy clay soil it really easy to do once you get used to the idea.

    Bookmark   December 11, 2009 at 9:09PM
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There's all kinds of justification for amending soils - no one is denying that issue. However, whenever possible, amending should still be done to an entire planting area rather than to individual planting holes. This is even more important in heavy clay soils. Amending the planting hole in a clay soil will result in a bucket or bathtub effect - an area of enriched, moisture retentive soil enclosed by heavier, slow to drain soil. After a good rainfall, the planting hole is just a bucket of water that has nowhere to go!

The notion of mixing amended soil with indigenous soil to create a "bridge" soil is a fallacy. You are still dealing with different soil textures and therefore soil interface issues. The rule of thumb for backfilling ANY planting hole is what comes out goes back in.....without additions. If amending the entire area is not possible, add the amendments as a top dressing, not into the planting hole. They will still add to and improve soil conditions but from the plant's perspective, in a much more healthy and efficient manner.

This is a gardening concept that is difficult for many to comprehend. As several of the above links note, most nurseries/garden centers, landscapers, some horticulturists and even extension services adhere to the old practice of amending planting holes. It is not without cause that many of these also sell or recommend various amendments as well, many of which may be unnecessary. It is just how they do business. That doesn't mean it is correct or advised.

As a professional horticulturist, I've attended all manner of seminars where the speakers recommend old planting methods. Many of these are old school sort of folks that do not keep up with the changes in the methods and theories. They know what they know and that's that! I'd assume the speaker madeyna refers to is from that generation. Some of his recommendations are just incorrect. For one, you never disturb the soil in the bottom of the planting hole or add amended soil to the bottom of the hole before planting. Plant roots grow laterally, not straight down: they do not need loose or enriched soil beneath them and settling of disturbed/amended soil under the plant results in the plant eventually winding up below grade. Too deep planting - typically the result of this erroneous practice - is a primary cause of plant failure. When in doubt, always plant slightly higher than grade.

In clay soils, the planting hole should be very wide and shallow, so that the top of the rootball is slightly above grade. Loosening the sides (only) can assist the root spread. Backfill with the removed soil only, breaking it up as necessary. Mulch with whatever amendments you desire or think you need just to the top of the rootball. The wide, shallow planting hole will accommodate any poor drainage issues the clay soil may present and is easier to dig than a deeper hole.

And as to the interface issues created because of the container medium or the B&B clay rootball, it is also recommended to bare root plants before planting to eliminate this issue. This has the added advantage of visibly seeing and therefore correcting any circling or girdling roots which are very common with container grown stock and can easily lead to plant failure as well. But this is a notion that is even harder for most non-professionals to accept, although it makes perfect sense and there is also a great deal of evidence to support its validity.

Horticulture is a scientific course of study and changes in practices occur as testing and scientific evidence dictate the need, just as happens in any other science-based field of study. Holding on to outdated concepts just because "it's always been done this way" or "that's how the nursery told me to do it" serve no purpose and can be detrimental to the outcome.

    Bookmark   December 12, 2009 at 11:31AM
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Pam, big thanks!
Still reading.
Funny thing is that due to the heavy rocks contents in my soil I practice shallow planting for the last 4-5 years and both, plants and my back are happy about that :-)

    Bookmark   December 13, 2009 at 9:04PM
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George, you are entirely welcome!

Shallow, wide planting holes are a good thing and even more so if the soil is hard to work. In my former garden, I had a clean slate and created the garden from scratch, including amending and importing soil before any planting was done. My soil was so soft and fluffy and full of organic matter, I could dig with my bare hands. Everything thrived. In my new garden, already established and in amongst a lot of big trees, I don't have that luxury -- poor soil, lots of rocks and tree roots everywhere. So I plant everything high and mulch like crazy with compost. Eventually I should have decent soil to work with :-)

    Bookmark   December 14, 2009 at 10:44AM
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Gardengal....Wow! I have really enjoyed reading about the information you generously explained about soil amendment. It makes perfect sense to me. Basically in a nut shell....don't dig a flower pot. We have a lot of landscaping to do (back yard is all but done) mainly in the front and I am going to put your good advice to use. Thank you for taking the time to explain all of this. It is very useful information.
Best, Claire : )

    Bookmark   December 16, 2009 at 9:09PM
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Gardengal, I want to thank you also 3-1/2 years later. Your generous writing is so appreciated by those of us who have been gardening for many years but just by following old practices. Yesterday i planted three very large and three small Limelights and wish to God I had read this first. I am 72 and in no mood to dig those guys up but i bet I will anyway. Tomorrow. Actually I should have known better because some expert Hosta growers have been expounding the what-come-out-goes-back-in theory for years. I just forgot.
Thank you again. Hope you see this.

    Bookmark   May 6, 2013 at 3:23PM
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