Leaf Drop Can Harbor Insects, Disease

Q. Is it best to remove all the leaves from the soil after they have fallen or leave them on the soil as mulch for fruit trees? My concern is about insects that might stay there during the winter and cause damage in the spring.

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From reader

A. In a very general sense I have two concerns regarding leaving undecomposed leaves at the base of plants. You are right, the first one is insect problems and it's very possible some insects will overwinter under loose leaves that have not decomposed.

            Remember to pick up any fruit lying on the ground as well. Do not leave dried fruit on fruit trees because of potential insect problems.

            One insect in particular is the grape leaf skeletonizer. Don’t leave debris around the base of grape plants because of this insect problem. Make sure leaves are shredded or composted. If you have one, lawnmowers are efficient leaf shredders.

The second potential problem is disease. There are a few diseases that cut overwinter on fallen leaves. It is always better to compost or shred leaves if you're going to leave them around the base of plants. Sanitation in the garden is important.

Fungus Gnats Problem in Potting Soils

Q. It seems no matter what brand of potting soil, I have to bake it before I can use it. If I don't, I get hundreds of tiny flies that hatch and swarm. I put out water to catch and drown them. I spray insect oil on top of the soil several times a day. I'm so afraid I'll kill the plants.

A. Yes, fungus gnats in particular are a big problem in potting soils used for houseplants. The younger generations feed off of both decaying plants and soft, succulent living roots. They aren’t very particular about what they feed on, living or dead, so long as it is soft, juicy and tender.

If fungus gnats are extremely happy in their environment they will multiply very rapidly and cause poor growth and stunting. Besides, they are pesky and a nuisance inside the house. If potting soil is sterilized by the manufacturer using a heat treatment it should kill all of the fungus gnats and should pose no problem.

Control fungus gnats with organic pest control products such as beneficial nematodes that go after their destructive larvae and a bacterium is also available with a similar result. You should be able to find these products in your local nursery or garden center.

Yellow or blue sticky traps also work. I received this video on how to make yellow sticky traps from a friend.

Another effective method is to sterilize this potting soil yourself by placing it, moistened, into a clear plastic bag and let it bake in the sun. Temperatures need to get up to about 160 F for at least 30 minutes for good control.

Another option is to apply pyrethrin sprays to the soil and water it in.

More information about fungus gnats from the University of California

Am I Applying Too Much Nitrogen?

Q. We fertilize all of our bushes early spring and mid fall with a 5-10-5 liquid plant tonic.  Hopefully this isn't too much nitrogen.

This concentrated fertilizer
 has no nitrogen but very
high percentages of phosphorus
and potassium

A. There are two ways to look at this question. Applying too much nitrogen can mean either applying it too often or applying too much in a single application. If you follow label directions, the amount of nitrogen applied should be correct. Apply nitrogen as often as 8 weeks apart if you want continuous growth.

             Applying excessive amounts of nitrogen can damage plants or cause excessive growth of leaves and stems.

            When judging how much fertilizer to apply and how often, observe the plant. If the plant does not have good growth or the flower size and numbers have diminished, apply fertilizers or “plant tonics” if you prefer.

Applying too much nitrogen, the first number, is not normally a long-term problem. The potential long-term problem involves the over application of the second or middle number, phosphorus. 

Phosphorus stays bound in many soils for much longer periods of time than nitrogen. Apply fertilizers containing high levels of phosphorus (middle number) less often than fertilizers that contain high levels of nitrogen.

As a general rule of thumb, apply fertilizers containing phosphorus once a year to established plants, two weeks before flowering. The rest of the time use high nitrogen fertilizers.

The exception is at planting time. Every time seed or transplants go in the ground, apply a high phosphorus fertilizer. Before planting, mix high phosphorus fertilizers in the soil to the same depth the roots will grow.

Bagged compost can be hard to find. Most
bagged products that contain compost are
soil mixes, not straight compost.

Consider using compost instead of a mineral fertilizer. I am talking about compost which is harder to find, not a soil mix that contains compost. Most composts have a good balance of nitrogen, phosphorus and potassium and can be used as a substitute for mineral fertilizers.

Composts provide plants with a lot of minor elements not found in mineral fertilizers. Compost lowers soil alkalinity, stimulates good soil microorganisms and provides humus or “black gold” to the soil. It is a very powerful addition to desert soils.

Some Houseplants Need Their Roots Pruned

Q. We repotted a house plant that was constantly wilting and that seems to help. Is it possible that the plant is too large and needs a “haircut”?

A. Potted plants benefit from root pruning as well as top pruning if it is possible to do so. I guess we could call this a “haircut”. Both the top of the plant and its roots should be “in balance” with each other. We sometimes refer to this as a plant’s “root to shoot ratio”.

            More about root to shoot ratios

Not all plants can be root pruned and top pruned to make them smaller so they perform better in their container. Some plants will need a larger container. Some plants are made smaller by “dividing” the plants. Plants that require division are removed from their container and separating a “clump” of plants into individual plants with a sharp, sanitized knife or pruning saw. The cut ends are dipped into a fungicide before replanting.

Some plants require renewal through propagation. Typically these plants are propagated using methods such as stem cuttings, leaf cuttings, root cuttings or layering. These specialized propagation techniques are specific to certain plants. The Internet is a good source of information on propagating interior or houseplants.

Propagation of Houseplants