Saturday, January 9, 2021

Seed Germination - Nothing's coming up!





Planting a vegetable garden has a variety of challenges. But how disappointing when the seeds are planted and nothing comes up! Here are primary reasons for poor seed germination.

 1. Planting depth – small seeded vegetables like carrots and onions can easily be planted too deep. If planted too close to the surface, the seed can easily be washed away during heavy rains. Consult the seed packet for proper planting depths. 

2. Temperature – some seed need the soil temperature to warm up before they will adequately germinate, while other cool season vegetables like onions, germinate best in a cool soil. 

 3. Water -  too little and seeds will not germinate or will dry up after germination. With too much water, seeds and seedlings can die from lack of oxygen or be overcome by fungi. Even under normal moisture conditions, different species of fungi and bacteria can cause seeds to rot.  Seeds and seedlings should always be kept moist, but the soil should not remain soaking wet.  

 4. Soil pH - each vegetable seed as well as other plants will have its range in which it will germinate best. 

5. Herbicide residues -  may still be active in the soils causing poor germination.  A high fertilizer content in the soil could also hamper germination. 

6.  Insects will often feed on  seeds and young seedlings.

7. And finally, the seed itself! Poor seed germination can particularly be a problem in humid climates.  Relative humidity influences the moisture content of the seed. The higher the moisture content, the lower the germination rate will be, especially, after one year.


As a general rule, vegetable and flower seeds can be kept for one year without appreciable decrease in germination. Storage, however, may be extended to 10 or more years under proper conditions; seed moisture and temperature are the most important factors. The drier the seeds, the longer they will store.

In humid climates such as Hawaii, storing vegetable seeds in a dry, sealed glass container should keep most seeds viable for a year. For longer storage, place seeds in a moisture-proof container and store in the refrigerator.

Longevity of vegetable seeds will also vary depending on the species.  For example, collards, cucumber, endive, radish and water cress produce some of the hard seeds, rated as lasting 5 years. Next come beets, cabbage, cauliflower, eggplant, kale, mustard, squash, swiss chard, tomato, turnip, watermelon, rated at a 4 year longevity; asparagus, beans,  carrot,  celery,  chinese cabbage, New Zealand spinach, pea, and spinach, at 3 years; leek,  sweet corn, okra and pepper are rated at 2 years; and finally, the last group is rated at a mere 1 year longevity - lettuce, parsley, parsnip and onion.



Photo: Purdue University

Friday, November 6, 2020

Avocado trees - Spots on the leaves?





Originating in Mexico, the persea mite, Oligonychus perseae, was found in California in the early 1990’s and probably came to the Big Island of Hawaii in the early 2000’s. The main host for the mites is avocado trees. Some varieties are more susceptible than others; the Lamb-Haas is considered least susceptible. Many ornamentals and weeds also host the persea mite.  The adult mites have eight legs and an oval body with two dark reddish eye spots near the head end of the body. They lack antennae and body segmentation.


                                                            

Mites feed both on the fruit and leaves, most notably on the underside of leaves. The damage appears as small, circular, yellow to brown spots. Mite colonies often reside against the protruding midrib vein of the underside of the leaves. They produce a dense, silk webbing of protection over them. The webbing resembles a silvery spot on the underside of the leaf, often seen glistening under sunlight.  


Premature leaf drop will begin once the feeding damage to the leaves reaches and exceeds 10% of the surface area. This can lead to sunburned bark and fruit; yields have been reduced up to 20 %.  Trees will recover from the defoliation by producing a new flush. But repeated dropping of the foliage will severely stress the tree.


In the home garden, some mites can be tolerated. Several species of natural enemies often reduce the population of the mites. If numbers get too high, horticultural oils will suppress the mite population. It is important that the spray solution contact the undersides of the leaves, where mites are located. Where feasible, spraying the undersides with a forceful stream of water can also reduce mite populations. By using hydrated lime or diluted white latex paint, a whitewashing on the trunk and major limbs will help protect bark and wood from sunburn after a premature leaf drop.

Photo of Mites: UC IPM Program

Friday, September 25, 2020

Pickleworm

                                                           
  
Why does pumpkin squash ( kabocha/kombucha, Japanese pumpkin) grow into a nice vine, but many of the young fruit turn yellow and drop off? 

This sounds like a case of an insect pest called the pickleworm( Diaphania nitidalis).  This moth was discovered in O`ahu in 2003, and by 2005 had spread to Kauai, Maui and the Big Island.  It attacks many cucurbits such as cucumber, squash, pumpkin, zucchini and cantaloupe.  Squash seems to be the most susceptible. The caterpillars like to feed on the blossoms and afterward will burrow into the developing fruit. The adult moths are colored with yellow and brown and have a purplish sheen. 


Some insecticides are registered on cucurbits and have been successful, with diligent spraying, in commercial crops.  One organic insecticides that may be effective on the pickleworm is known as Bt, (Bacillus thuringiensis).  It is actually a bacterium which is deadly to the moth larvae, but not to humans.  One problem is coverage; the spray adheres to the surface of the leaves and blooms, while the caterpillar is on the inside of the bloom and young fruit.

                                                              

It has been difficult for the homeowner to control the pickleworm. One method is exclusion.  Place screening material over the plants, or individual blooms.  This will keep the moth out and prevent her from laying eggs.  The problem however, is that the cover would also exclude honeybees.  Honeybees are important for pollinating many of the cucurbit crops.  A lack of pollination can be a cause of blossom drop.  Because the moth is a night flyer, you can cover the crop at dusk and uncover it during the day.  This will allow honeybees to visit the plant during the daylight hours, but keep the moths out at night.  Admittedly this is quite a tedious process.

Alternatively, since the moth is relatively large, a wingspread of about one inch,  ¾ inch bird netting can be used to build a permanent structure over the plants. This will exclude the moth while allowing bees to freely move through the netting.

There are a few squash varieties which show some resistance to the moth.  They are Butternut 23, Summer Crookneck, Early Prolific Straightneck and Early Yellow Summer Crookneck.  There have also been reports from other areas of the country, that the early crops have less damage (may not be applicable to Hawaii).

Recently a company named High Mowing Organic Seeds has released a new variety of zucchini called Partenon.  It is a parthenocarpic variety which means that fruit develops without pollination. Thus this variety can be grown in places that  exclude pollinators (honeybees) such as a greenhouse or netted structure.   Partenon is also tolerant to powdery mildew.  (www.highmowingseeds.com )