Asparagus (Asparagus officinalis L.) is produced on approximately 1,730 hectares (2002) in Nagano Prefecture in Japan, of which about 10-15 % needs to be replanted every year. Replant of asparagus often results in lower spear yields and an extensive damage of young plants. One of the causes of these phenomena is allelopathy. In Nagano Prefecture, the mitigation method of allelopathy, using activated charcoal flowable (activated charcoal F) has gained popularity widely since 2001. Today, the sprinkling, dipping of nursery plants and an irrigation method using activated charcoal F are carried out to mitigate allelopathy in asparagus. However the treatment with activated charcoal F is limited to replanted fields. In this study, we examined the effects of injection period and concentration on the deep placement method of activated charcoal F on the growing period of asparagus. The results of local adaptability tests on 1-4 years old asparagus plants showed that activated charcoal F (using a 25 to 100-fold dilution) was effective in mitigating allelopathy and stimulated growth in asparagus. But on 6-7 years old asparagus plants the effect of activated charcoal F was lower due to the big root system. Suited treatment period was the mother fern stage because spacing between plants can be observed. The same as for the sprinkling, dipping of nursery plants and an irrigation method, can be concluded that deep placement method of activated charcoal F appears to be effective in mitigating the allelopathy of asparagus in this field experiment.