Horticulture Guy

Q. My friend says there is no such plant as ‘Impatiens Grandelifera’.  Also, called ‘policemens helmets’.  I discovered them 9 years ago. Thinking they were weeds I pulled a few, took them to the master Gardner at Lakewold Gardens.  She had just received seeds from a friend.  They grow to 5 ft. tall with pink blossoms resembling snapdragons.  In the fall pods form from the blossoms and when ripe explode, scattering the seed for the following spring.  Can you find any info on this?  Is this the correct name?  Your help is appreciated.  Thanks, Ruth Offutt - University Place

A.  Perhaps your friend is a stickler for spelling since I have seen this species also written as I. grandulifera but I believe the proper Latin binomial is I. glandulifera.  Policeman’s Helmet is a common name for this herbaceous annual from India and the western Himalayas. They prefer moist sites and have the capacity to grow up to 10 feet tall and eject their seeds as far as 20 feet.  As pretty as they are the Washington State Noxious Weed Control Board lists them as a Class B Weed so I would not suggest planting them or sharing seeds.  For more information about this plant see the Washington State Noxious Weed Control Board website: http://www.nwcb.wa.gov/weed_info/Impatiens_glandulifera.html

-- HG

Q. We are seeking your advice concerning a problem with neighbors who share a common fence-line who have planted invasive plants (bamboo) against this fence.  The large strong roots are invading our yard and landscape.  Recently they planted several more bamboo trees, which will eventually invade the rest of our yard.  Do you know how we can successfully stop these roots? Are there any property laws that would deal with this situation?  We would appreciate any advice you can give us.  Concerned Homeowners Ron and Barbara Bishop - Tacoma

A.   I can sympathize with your situation.  Running bamboo can be a very invasive plant when not contained.  There are clumping types of bamboo that are not considered invasive.  Unfortunately you don’t know which the new bamboo is. Since the existing plants are running you may have to assume the new bamboo is as well.  Periodic root pruning of the fence line with a shovel is a good short-term solution but will get old very fast especially where existing plantings are located.  The typical solution to controlling running bamboo is to install a root or rhizome barrier.  These can be made from metal, plastic and sometimes concrete.  They provide a barrier below the soil line any where from 2-3 feet deep and usually above the soil line up to 6 inches.   Jade Mountain Bamboo (548-1129) in Tacoma can help you select the best barrier.  I am not cognizant of the current laws in Tacoma regarding your situation so you will have to investigate that yourself.  

-- HG

Q. I have been treating my thundercloud plums for an aphid infestation. We have been using neem oil and insecticidal soap once a week for about two to three weeks. Now, the trees are losing a significant number of leaves. Is this because the aphids drained the leaves of moisture and they have dried and fallen off? Is it possible that we burned the leaves with the neem oil? Is the tree simply in shock? Do we need to try a different pest-control treatment? Do we need to prune at all? Will our trees recover from this? Please help Feather Asmussen - Everett, WA

A. The condition where leaves are lost or damaged due to the spraying of a chemical is called phytotoxicity. This can often happen with summer oil sprays especially when the plants are under stress from lack dry soils and/or when temperatures get above 80 degrees. When you say you are spraying with neem oil and insecticidal soap it sounds like you are applying them simultaneously. This is not necessary as both products on their own will control aphids. More is not necessarily better. It is likely that the combination of the two ingredients that magnifies their phytotoxicity. I would back off to just using the insecticidal soap until you get the aphids under control. Of course the amount of concentrate added to a specific quantity of water should be as indicated on the product label. Increasing the amount does not improve the efficacy and can increase the chance of phytotoxicity. If the trees have not been defoliated before they will rebound and send out new leaves and therefore will likely recover.

-- HG

Q. Dear Horticulture Guy; Several years ago, we planted three New Zealand Flax plants which have done very well.  In fact, they have grown so large that they are now crowding the street, the adjacent plantings around them and they’re even crowding each other.  How do we control them?  Should they be divided, and if they can be, how does a person do this? The flaxes are so large, tough and fibrous, with extremely sharp edges and points on the tips. It seems it would be a daunting task.    Your suggestions would be most appreciated.  Thank you, Chuck and Linda Price - University Place, WA

A.  You are a victim of your success and plant selection.  There are two species of New Zealand Flax (or sometimes called Flax Lily) found in cultivation on the west coast of the United States.  The one you have selected is Phormium tenax which has leaves that can grow as long as 9 feet long.  They can get as wide as 6 feet and have flower stalks 15′ tall.   Yes you can divide these plants and it may be daunting to some that are used to more succulent perennials.  The proper tools would be a very sturdy shovel or perhaps even better a saw to get through the fibrous root system.  But this will only give you a temporary reprieve as they are fast growers and you will likely be back to their full size in a few years.  One option is to relocate the divisions to areas that can accommodate their full mature size.  The other option is to trade them to a friend who has space for them and get a smaller variety for your own property.  Phormium cookianum  (also P. cookianum) is a smaller species that has pendulous flower stalks (unlike P. Tenax which has erect flower stalks). There are small selections of this species and its hybrid with P. tenax that which only grows to about 3 feet tall and 4 feet wide.   The down side is that they are slightly less hardy than P. tenax.  

-- HG

Q. I am interested in using corn gluten on my lawn to prevent weeds from growing.  Can you provide some info on this, including which form is best to use, where to buy it, and which weeds it prevents (does it work on bind weed?).  Thanks.

A.  First off corn gluten does not work on existing weeds.  Gluten is a protein which when it functions properly dries out germinating seeds thus killing them.  Any plants that are growing in the application are will benefit from additional nitrogen.  This means both existing weeds and the lawn.  The original research was very promising but possibly over promoted by some distributors of the product. In addition I also believe that people were so happy to have an organic alternative for weed control (myself included) that they were willing to give it a shot.  There have not been any thorough studies on what weed seeds it works on in the PNW and preliminary trials were less than stellar.  If you would like more details Dr. Linda Chalker-Scott over at WSU Extension has written a great piece on this subject:

http://www.puyallup.wsu.edu/~Linda%20ChalkerScott/Horticultural%20Myths_files/Myths/Corn%

-- HG

Q. We are new to gardening in Washington where we get moss in our flowerbeds. After raking and breaking it up, is it important also to pick it up since so much mulch or soil clings to it? Rae Ann Merrill - Gig Harbor

A. Although moss is usually removed from lawns as well as garden beds I wouldn’t say removal from a garden bed is as important as from a lawn (unless you are starting plants from seed).  This is because the moss shades the lawn even as it dies after being removed.  What would be important is to determine if the conditions that promote the moss can be changed.  After removing the moss (I would suggest composting it and then returning it to the beds) cultivate the surface to promote drainage.  You will not likely see moss during the summer once you rake it out especially if the beds are sunny.  Compaction, excess moisture as well as excessively low or high soil pH and poor fertility of the soil can promote moss so anything you do to mitigate these conditions will help.  Optimal soil pH is between 6 and 7.

-- HG

Q. When we moved into our Lakewood/Steilacoom area home 5 years ago, we accidentally cut down a pear tree. The tree is growing back and needs to be trimmed/shaped. When is the best time of year to do this? I do not know what type of pear tree it is.  Thank you for your help. Susan Greer Lakewood, WA 

A.  I wonder if good ol’ George Washington used that as his defense with the cherry tree?  The real question here is whether or not it would be worth to try and reclaim this tree at this point.  Was this a fruiting pear or primarily an ornamental like a Callery Pear (Pyrus calleryana) or was it a fruiting pear?  If it was an ornamental pear or a standard (very large) fruiting pear then the new growth you are seeing would likely be the same as the growth you cut away. This is because the tree would likely be growing on its own roots.  If it was any other fruiting pear (dwarf, semi-dwarf etc.) it was likely grafted onto a different rootstock and you would not have the same tree you started with.  Upi would not get the fruit you expected. For example many fruiting pears are grafted onto the ornamental rootstock like the Callery Pear mentioned earlier.  Some pears can even be grafted onto Quince rootstock.  So you may not even get the same size tree as much of the dwarfing nature of rootstock comes from its relationship to the grafted tree and not the rootstocks innate dwarfness.  If in fact this is a tree you wish to salvage you will need to choose a few new leader.  The new growth may show a branch that seems to have reached up above the others.  Three leaders are usually sufficient.  This can be done in the summer.  Then winter pruning can take care of the rest of the shaping.  Pears are pruned to a modified leader system. 

-- HG

Q. I have a beautiful Rhoddy which is about 6ft. high. The last couple of years it has become quite leggy. There were a lot of blossoms this year, but grouped at the end of the branches, which seem to be pretty weak.  I know the time to prune it back is now!!  There is new growth in the center of the bush. The problem is we do not know where to make the cut. My husband is good at pruning, but he wanted to make sure he did this one right. We have had this rhoddy for years. Once again it is about 6ft. high and about 8ft. wide.  Please help!! I do not want to loose this bush.  Thank you Margaret Miller - Puyallup
A.  Well your concern is well founded because even professionals will approach pruning rhododendrons in different ways.  This is because different species react differently to pruning.  Some can be cut to a foot above the ground and send up a new shrub in a few years.  Others cut similarly will not send out new shoots and will never recover.  This can also happen when long branches are cut back.  Sometimes they resprout and other times they will not.  It would be difficulty to tell you the exact way to prune your rhododendron except for the fact that your rhododendron provided proof that of its ability to sprout lower on the branches. The location for pruning cuts is found by following the branch down to a place where it branches.  In your case you can also prune back branches just above the growth you have described “in the center of the bush”.  For many rhododendrons what you describe as leggy is really a desire of the rhododendron to move from being a large shrub to a small tree.  In some cases it is better to move the shrub than to fight with it. This is especially true when it is grown too close to a house or other plants.   

-- HG